Always add the en-US locale fallback for translations (bug 988376)

fix [bug 988376](https://bugzilla.mozilla.org/show_bug.cgi?id=988376)

This is based on PR #18 which can't be merged because of the "last chance
callback" requiring a too big performance hit because of the needed "group by".

In this PR, we still have most of the behavior described in #18, but without
the "last chance fallback":

Check the following locales in order
1/ the active language
2/ the model fallback provided by the ```get_fallback()``` method (if it exists)
3/ ```settings.LANGUAGE_CODE```