Bug 1522101 - Use `prettier --check` to validate formatting

Currently Prettier is used via the ESLint integration, which only supports
JS/JSX and not the other filetypes Prettier is able to format.

For now these additional filetypes are excluded by `.prettierignore`
entries, however these will be removed/fixes applied in later commits.

.travis.yml

@@ -15,6 +15,8 @@ matrix:
       script:
         # `yarn build` is tested as part of the Selenium job.
         - yarn lint
+        # `yarn lint` only checks the formatting of JS/JSX, this will also check CSS/HTML/JSON/Markdown/YAML.
+        - yarn format:check
         - yarn test
 
     - env: python2-linters

docs/code_style.md

@@ -38,7 +38,13 @@ We use Prettier for JS/JSX formatting and the [Airbnb](https://github.com/airbnb
 guide for non-style related best practices. Both are validated using ESlint (see Validating
 Javascript in the [Installation section](installation.html#validating-javascript)).
 
-We recommend that you [add Prettier to your editor/IDE](https://prettier.io/docs/en/editors.html)
+Prettier is also used to format JSON/CSS/HTML/Markdown/YAML. However these are not supported
+by ESLint, so instead are validated using Prettier's CLI. To manually check their formatting
+(as well as that of JS/JSX) using Prettier, run `yarn format:check`, or to apply formatting
+fixes run `yarn format`.
+
+However we recommend that you instead
+[add Prettier to your editor/IDE](https://prettier.io/docs/en/editors.html)
 and enable "format on save" for the most seamless development workflow.
 
 Imports in JS/JSX must be ordered like so (with newlines between each group):

package.json

@@ -98,6 +98,8 @@
   "scripts": {
     "build": "node ./node_modules/webpack/bin/webpack.js --mode production",
     "build:dev": "node ./node_modules/webpack/bin/webpack.js --mode development",
+    "format": "node ./node_modules/prettier/bin-prettier.js --write \"**/*.{css,html,js,jsx,json,md,yaml,yml}\"",
+    "format:check": "node ./node_modules/prettier/bin-prettier.js --check \"**/*.{css,html,js,jsx,json,md,yaml,yml}\"",
     "heroku-postbuild": "yarn build",
     "lint": "node ./node_modules/eslint/bin/eslint.js --cache --report-unused-disable-directives --max-warnings 0 --format codeframe --ext js,jsx \".*.js\" \"*.js\" ui/ tests/ui/",
     "start": "node ./node_modules/webpack-dev-server/bin/webpack-dev-server.js --mode development",