Make our own screenshotting script instead of using huxley

Summary:
Create our own screenshotting script which takes screenshots. This
improves over huxley for a couple reasons:
 - It makes the screenshots the correct size (for some reason, huxley struggles
   with this).
 - Its configuration matches more with what we want (we don't need multiple
   screenshots or interaction, we just want a single static shot)
 - It runs faster

I also changed the docs to reflect this change.

Test Plan:
 - Make sure all of the tests that were in the Huxleyfile are now in ss_data.json
 - Run the screenshotter docker
 - Make sure all of the images look reasonable and don't change (except
   sometimes the Lap test, which has some strange pixel-positioning
   differences...)

Reviewers: kevinb

Reviewed By: kevinb

Differential Revision: https://phabricator.khanacademy.org/D16731

test/screenshotter/ss_data.json

@@ -0,0 +1,36 @@
+{
+    "Accents": "http://localhost:7936/test/screenshotter/test.html?m=\\vec{A}\\vec{x}\\vec x^2\\vec{x}_2^2\\vec{A}^2\\vec{xA}^2",
+    "Baseline": "http://localhost:7936/test/screenshotter/test.html?m=a+b-c\\cdot d/e",
+    "BasicTest": "http://localhost:7936/test/screenshotter/test.html?m=a",
+    "BinomTest": "http://localhost:7936/test/screenshotter/test.html?m=\\dbinom{a}{b}\\tbinom{a}{b}^{\\binom{a}{b}+17}",
+    "Colors": "http://localhost:7936/test/screenshotter/test.html?m=\\blue{a}\\color{%230f0}{b}\\color{red}{c}",
+    "DeepFontSizing": "http://localhost:7936/test/screenshotter/test.html?m=a^{\\big| x^{\\big(}}_{\\Big\\uparrow} + i^{i^{\\Huge x}_y}_{\\Huge z} + \\dfrac{\\Huge x}{y}",
+    "DelimiterSizing": "http://localhost:7936/test/screenshotter/test.html?m=\\bigl\\uparrow\\Bigl\\downarrow\\biggl\\updownarrow\\Biggl\\Uparrow\\Biggr\\Downarrow\\biggr\\langle\\Bigr\\}\\bigr\\rfloor",
+    "DisplayMode": "http://localhost:7936/test/screenshotter/test.html?m=\\sum_{i=0}^\\infty \\frac{1}{i}&pre=pre&post=post&display=1",
+    "DisplayStyle": "http://localhost:7936/test/screenshotter/test.html?m={\\displaystyle\\sqrt{x}}{\\sqrt{x}}{\\displaystyle \\frac12}{\\frac12}{\\displaystyle x^1_2}{x^1_2}",
+    "Exponents": "http://localhost:7936/test/screenshotter/test.html?m=a^{a^a_a}_{a^a_a}",
+    "FractionTest": "http://localhost:7936/test/screenshotter/test.html?m=\\dfrac{a}{b}\\frac{a}{b}\\tfrac{a}{b}",
+    "Functions": "http://localhost:7936/test/screenshotter/test.html?m=\\sin\\cos\\tan\\ln\\log",
+    "GreekLetters": "http://localhost:7936/test/screenshotter/test.html?m=\\alpha\\beta\\gamma\\omega",
+    "KaTeX": "http://localhost:7936/test/screenshotter/test.html?m=\\KaTeX",
+    "Lap": "http://localhost:7936/test/screenshotter/test.html?m=ab\\llap{f}cd\\rlap{g}h",
+    "LeftRight": "http://localhost:7936/test/screenshotter/test.html?m=\\left( x^2 \\right) \\left\\{ x^{x^{x^{x^x}}} \\right.",
+    "LeftRightListStyling": "http://localhost:7936/test/screenshotter/test.html?m=a+\\left(x+y\\right)-x",
+    "LeftRightStyleSizing": "http://localhost:7936/test/screenshotter/test.html?m=+\\left\\{\\rule{0.1em}{1em}\\right.x^{+\\left\\{\\rule{0.1em}{1em}\\right.x^{+\\left\\{\\rule{0.1em}{1em}\\right.}}",
+    "NestedFractions": "http://localhost:7936/test/screenshotter/test.html?m=\\dfrac{\\frac{a}{b}}{\\frac{c}{d}}\\dfrac{\\dfrac{a}{b}}{\\dfrac{c}{d}}\\frac{\\frac{a}{b}}{\\frac{c}{d}}",
+    "NullDelimiterInteraction": "http://localhost:7936/test/screenshotter/test.html?m=a \\bigl. + 2 \\quad \\left. + a \\right)",
+    "OpLimits": "http://localhost:7936/test/screenshotter/test.html?m={\\sin_2^2 \\lim_2^2 \\int_2^2 \\sum_2^2}{\\displaystyle \\lim_2^2 \\int_2^2 \\intop_2^2 \\sum_2^2}",
+    "Overline": "http://localhost:7936/test/screenshotter/test.html?m=\\overline{x}\\overline{x}\\overline{x^{x^{x^x}}} \\blue{\\overline{y}}",
+    "PrimeSpacing": "http://localhost:7936/test/screenshotter/test.html?m=f'+f_2'+f^{f'}",
+    "RlapBug": "http://localhost:7936/test/screenshotter/test.html?m=\\frac{\\rlap{x}}{2}",
+    "Rule": "http://localhost:7936/test/screenshotter/test.html?m=\\rule{1em}{0.5em}\\rule{1ex}{2ex}\\rule{1em}{1ex}\\rule{1em}{0.431ex}",
+    "SizingBaseline": "http://localhost:7936/test/screenshotter/test.html?m={\\tiny a+b}a+b{\\Huge a+b}&pre=x&post=M",
+    "Sizing": "http://localhost:7936/test/screenshotter/test.html?m={\\Huge x}{\\LARGE y}{\\normalsize z}{\\scriptsize w}",
+    "Spacing": "http://localhost:7936/test/screenshotter/test.html?m=^3+[-1][1-1]1=1(=1)\\lvert a\\rvert~b",
+    "Sqrt": "http://localhost:7936/test/screenshotter/test.html?m=\\sqrt{\\sqrt{\\sqrt{x}}}_{\\sqrt{\\sqrt{x}}}^{\\sqrt{\\sqrt{\\sqrt{x}}}^{\\sqrt{\\sqrt{\\sqrt{x}}}}}",
+    "SupSubCharacterBox": "http://localhost:7936/test/screenshotter/test.html?m=a_2f_2{f}_2{aa}_2{af}_2",
+    "SupSubHorizSpacing": "http://localhost:7936/test/screenshotter/test.html?m=x^{x^{x}}\\Big|x_{x_{x_{x_{x}}}}\\bigg|x^{x^{x_{x_{x_{x_{x}}}}}}\\bigg|",
+    "SupSubOffsets": "http://localhost:7936/test/screenshotter/test.html?m=\\displaystyle \\int_{2+3}x f^{2+3}+3\\lim_{2+3+4+5}f",
+    "Text": "http://localhost:7936/test/screenshotter/test.html?m=\\frac{a}{b}\\text{c~ {ab} \\ e}+fg",
+    "VerticalSpacing": "http://localhost:7936/test/screenshotter/test.html?pre=potato<br>blah&post=<br>moo&m=x^{\\Huge y}z"
+}

test/screenshotter/test.html

@@ -0,0 +1,32 @@
+<!doctype html>
+<html>
+  <head>
+    <title>Huxley test</title>
+    <script src="/katex.js" type="text/javascript"></script>
+    <link href="/katex.css" rel="stylesheet" type="text/css">
+    <style type="text/css">
+      #math, #pre, #post {
+        font-size: 4em;
+      }
+    </style>
+  </head>
+  <body>
+    <span id="pre"></span>
+    <span id="math"></span>
+    <span id="post"></span>
+    <script type="text/javascript">
+      var query = {};
+      var re = /(?:^\?|&)([^&=]+)(?:=([^&]+))?/g;
+      var match;
+      while (match = re.exec(window.location.search)) {
+          query[match[1]] = decodeURIComponent(match[2]);
+      }
+      var mathNode = document.getElementById("math");
+      katex.render(query["m"], mathNode, {
+          displayMode: !!query["display"]
+      });
+      document.getElementById("pre").innerHTML = query["pre"] || "";
+      document.getElementById("post").innerHTML = query["post"] || "";
+    </script>
+  </body>
+</html>