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

dockers/Screenshotter/README.md

@@ -0,0 +1,29 @@
+### How to generate screenshotter images
+----------------------------------------
+
+Now you too can generate screenshots from your own computer, and (hopefully)
+have them look mostly the same as the current ones! To start, make a docker
+image from the included Dockerfile using a command like
+
+    docker build --tag=ss .
+
+from within this directory (note you need to have docker installed and running
+for this to work). This will build a docker image with the `ss` tag, which you
+can then use to run dockers based on it.
+
+This Dockerfile is set up such that it will run everything and generate all the
+screenshots when the docker is run, so no interactive input is required. All
+that you need to do is mount the KaTeX directory you want to test into the
+`/KaTeX` directory in the docker, and run the `ss` docker, like so:
+
+    docker run --volume=/your/KaTeX/:/KaTeX ss
+
+The `--volume=/your/KaTeX:/KaTeX` switch mounts your KaTeX directory into the
+docker. Note this is a read-write mounting, so the new screenshots will be
+directly placed into your KaTeX directory.
+
+Since this docker is very self-contained, there should be no need to do
+interactive management of the docker, but if you feel the need, you can read the
+General Docker Help section of the MathJaxFonts docker readme.
+
+That's it!