Use translatable strings

Move all translations related to the theme from fun-apps to this
repository.  Previously, these files were included in fun-apps. It makes
more sense to include them directly in the theme and to point to them
via the common settings file of fun-apps.

Also, flag all strings as translatable. Finally, translate
these strings in django.po.

The localization file can now be maintained via the makemessages_fun
command (in fun-apps).