Internationalization support

Hi,
as per #692, this is an approach to add internationalization support to GitLab.
Not all pages are yet translated, most notably admin and help pages, but this can be done later.
I'm willing to maintain translations is a separate repository and, as it was suggested, use Transifex to collect translations.
As to concerns that this could slow down development I think this won't be the case if only English version will be supported, additionally it will simplify scenarios where a helper method was used to repeat the same message.