Make ESLint configuration compatible with Pronto

pronto-eslint depends on eslintrb, which expects the `.eslintrc` file
in JSON format. So we're migrating the `.eslintrc.yml` file to JSON and
introducing a symbolic link so the `.eslintrc` file points to
`.eslintrc.json`.

We could also use pronto-eslint_npm, but unfortunately it's not
maintained anymore and the latest version is not compatible with Pronto
0.11.

.eslintrc.json

@@ -0,0 +1,115 @@
+{
+  "env": {
+    "browser": true,
+    "es6": false
+  },
+  "extends": "eslint:recommended",
+  "globals": {
+    "$": "readonly",
+    "App": "readonly",
+    "AmsifySuggestags": "readonly",
+    "annotator": "readonly",
+    "c3": "readonly",
+    "CKEDITOR": "readonly",
+    "L": "readonly",
+    "Turbolinks": "readonly"
+  },
+  "parserOptions": {
+    "ecmaVersion": 5
+  },
+  "rules": {
+    "array-bracket-spacing": "error",
+    "array-callback-return": "error",
+    "block-spacing": "error",
+    "brace-style": "error",
+    "comma-spacing": "error",
+    "computed-property-spacing": "error",
+    "curly": "error",
+    "dot-notation": "error",
+    "eol-last": "error",
+    "eqeqeq": [
+      "error",
+      "always",
+      {
+        "null": "ignore"
+      }
+    ],
+    "func-call-spacing": "error",
+    "indent": [
+      "error",
+         2
+    ],
+    "key-spacing": "error",
+    "keyword-spacing": "error",
+    "linebreak-style": "error",
+    "lines-between-class-members": "error",
+    "max-len": [
+      "warn",
+         {
+           "code": 110
+         }
+    ],
+    "no-array-constructor": "error",
+    "no-console": "error",
+    "no-multi-spaces": "error",
+    "no-multiple-empty-lines": [
+      "error",
+         {
+           "max": 1
+         }
+    ],
+    "no-param-reassign": "error",
+    "no-shadow": "error",
+    "no-spaced-func": "error",
+    "no-tabs": "error",
+    "no-trailing-spaces": "error",
+    "no-void": "error",
+    "no-whitespace-before-property": "error",
+    "object-curly-spacing": [
+      "error",
+      "always",
+      {
+        "objectsInObjects": false
+      }
+    ],
+    "padded-blocks": [
+      "error",
+      "never"
+    ],
+    "quotes": [
+      "error",
+      "double",
+      {
+        "avoidEscape": true
+      }
+    ],
+    "semi": [
+      "error",
+      "always"
+    ],
+    "semi-spacing": "error",
+    "space-before-blocks": "error",
+    "space-before-function-paren": [
+      "error",
+      "never"
+    ],
+    "space-in-parens": "error",
+    "space-infix-ops": "error",
+    "space-unary-ops": "error",
+    "spaced-comment": [
+      "error",
+      "always",
+      {
+        "markers": [
+          "="
+        ],
+        "exceptions": [
+          "-"
+        ]
+      }
+    ],
+    "strict": "error",
+    "switch-colon-spacing": "error",
+    "yoda": "error"
+  }
+}