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