Patched processing of fields derived from translated fields.

Fields that need to be generated for all languages are:
* Slugged.slug,
* MetaData.description,
* Displayable.short_url,
* Page.titles.
Moreover, when generating description one needs to be careful not to
use existing translated description.

Factored out two helpers that may be useful for similar tasks in user
code. The disable_fallbacks() context prevents getting value from a
default language when one for the active one is missing, useful when
you need to know if a value in the current language is available.
The for_all_languages() function executes a function with each of the
available languages activated in turn.

The for_all_languages() function and the disable_fallbacks() manager
are used outside of USE_MODELTRANSLATION conditions, so tests with
enabled and disabled content translation are needed for them.
Model.save() should not use any more queries with content translation
than without it, query counts in both save performance tests should be
updated in unison.

TODO: Propose light changes to trans_null upstream.
TODO: Investigate Page.save() queries count on master.
Integrated modeltranslation tabbed admins.

Based all admins of models registered for translation 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.TranslationModelAdmin
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.

Keyword 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.
The latter is recomputed on change for all languages as with other
derived attributes.

Page admin allows entering keywords in the active language only (*),
so a new KeywordAdmin is exposed separately to make it possible to
translate keywords (under Content menu).

(*) There doesn't seem to be any sensible way to inline translations.
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.