Streamline javascript translation by improving translationRunner (#2808)

* package.json: Add "build:*" targets

* Improve react-intl-translations-manager workflow.
  * Added "build:production" to build production bundle.
  * Added "build:development" to build development bundle.

* Fix json translation files

* Run `yarn manage:translations` to fix translation files.
* Fix `pl.json` for syntax error.

* translationRunner: auto detect existing languages

* Auto detect existing rfc5646 language tag in *.json filenames
  in `app/javascript/mastodon/locale` folder. No need to manually
  define every new language in the languages array here.

* translationRunner: add more functionality

* Allow script user to specify language code to check.
* Added available language check.
* Added --force flag to force creation of unexists language.
* Added --help flag and help messages.

* gitignore: ignore npm-debug.log

* Fix webpack error if NODE_ENV is not defined

Default to use 'development' in config/webpack/configuration.js

package.json

@@ -3,6 +3,8 @@
   "license": "AGPL-3.0",
   "scripts": {
     "postversion": "git push --tags",
+    "build:development": "NODE_ENV=development yarn webpack -- --config config/webpack/development.js",
+    "build:production": "NODE_ENV=production yarn webpack -- --config config/webpack/production.js",
     "manage:translations": "node ./config/webpack/translationRunner.js",
     "start": "babel-node ./streaming/index.js --presets es2015,stage-2",
     "storybook": "start-storybook -p 9001 -c storybook",
@@ -113,6 +115,7 @@
     "eslint-plugin-jsx-a11y": "^4.0.0",
     "eslint-plugin-react": "^6.10.3",
     "jsdom": "^9.11.0",
+    "minimist": "^1.2.0",
     "mocha": "^3.2.0",
     "react-intl-translations-manager": "^5.0.0",
     "webpack-dev-server": "^2.4.5"