Lint .less files with stylelint (#925)

Lint .less files with stylelint. Fix #921

In particular, prevent tabs.

.flowconfig

@@ -5,6 +5,7 @@
 
 [libs]
 flow-typed
+./node_modules/stylelint/decls
 
 [lints]
 

.stylelintrc

@@ -0,0 +1,6 @@
+{
+    //"extends": "stylelint-config-standard",
+    "rules": {
+        "indentation": 4,
+    }
+}

package.json

@@ -47,13 +47,15 @@
     "pre-commit": "^1.2.2",
     "selenium-webdriver": "^2.48.2",
     "sri-toolbox": "^0.2.0",
+    "stylelint": "^8.2.0",
+    "stylelint-config-standard": "^17.0.0",
     "uglify-js": "~2.7.5",
     "webpack": "^3.6.0",
     "webpack-dev-server": "^2.8.2"
   },
   "bin": "cli.js",
   "scripts": {
-    "lint": "eslint katex.js server.js cli.js src test contrib dockers",
+    "lint": "eslint katex.js server.js cli.js src test contrib dockers && stylelint static/fonts.less static/katex.less",
     "flow": "flow",
     "jest": "jest",
     "coverage": "jest --coverage",

static/katex.less

@@ -294,7 +294,7 @@
             text-align: center;
         }
 
-       .frac-line {
+        .frac-line {
             display: inline-block;
             width: 100%;
             border-bottom-style: solid;