first attempt to serve correct localized js files as rails pipeline assets

Hi, this is a first attempt to implement locale switching into Discourse. I removed translations.js.erb (from both discourse/ and admin/) and instead created individual en.js.erb, fr.js.erb, etc. files in app/assets/javascripts/locales.

Now, it's possible to change locale simply by `config.i18n.locale = :fr` in config/application.rb.

Note that setting the locale on the JS side is a little bit tricky. I loaded the correct strings js file at the very beginning of application.html.erb, but you can't set `I18n.locale = xxx`, because the i18n library is not yet initialized and it will be overriden back to 'en'. So I hacked this place to save the current locale to a `window.currentLocale` variable and at the right place (right after I18n is initialized), it is loaded into `I18n.locale`. Also note that you can't simply set the locale *after* all the scripts, because some of them needs l10n strings already during their init (doing it will bring subtle bugs, e.g. some strings will be missing).

I also merged the "admin" strings into the strings files, because there were only a few strings. Not sure if this is better or not, but it's easy to change back.

Comments and changes are welcome.