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

@@ -0,0 +1 @@
+.eslintrc.json

.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"
+  }
+}

.eslintrc.yml

@@ -1,85 +0,0 @@
-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