Integrated modeltranslation tabbed admins.

Based all admins that may manage models with translated fields on
appropriate modeltranslation admins and included scripts needed
for displaying translations as tabs in the base admin template.

You may derive your admins from mezzanine.core.TranslationAdmin
even if you don't use content translation (at the moment).

Minor fixes needed were:
* rectifying extra copies of translation fields in forms (inserted in
  PageAdmin for proper Page subclasses);
* handling PageAdminForm posts with multiple values for slug_<lang>
  instead of just a single slug value (repeating slug propagation
  for all available languages).
Added a new update_generated_fields command.

The command simply resaves all models basing on anything that may have
translated generated fields (currently Slugged, MetaData and Page).

With some fields, one cannot always rely on translation fallbacks, as
they don't work with filtering / lookups. For example, if you create a
new project loading example fixtures, migrate and update translation
fields, you won't be able to browse most pages in any non-default
language until you give them a chance to generate translated slugs.

The command may also be used to reset all slugs (thus regenerating
them from titles).

TODO: Allow registering bases with generated translated fields.
Made modeltranslation fields on lists narrower, so a couple of
translations can fit.

This also gives a cleaner layout with vanilla admin in some places
(e.g. Cartridge Sale inline).

It would be nice to display just one translation for a whole inline
and control which one is displayed with the main language switch / tabs.
However, this would require a specific script for Mezzanine's dynamic,
sortable inlines. There's a sketch of a solution on the branch:
modeltranslation/feature/hide-inactive-untabbed.
Implemented keywords translation.

Assignments are intended to be the same for every language, thus only
Keyword.title and keywords_string (on concrete subclasses of MetaData)
are made translatable, with the latter recomputed on change for all
languages as with other derived attributes.

Page admin only allows entering keywords in the active language (there
doesn't seem to be any sensible way to inline translations), so
KeywordAdmin (under Content menu) is exposed separately to make it
possible to translate keywords.
Allowed testing of modeltranslation (and all other optional apps).

There was probably some reason for excluding these from testing,
however it could be unpleasantly surprising to check that all the
tests pass on a testing environment, but then something fails due
to an optional app being imported in production.