Automated modeltranslation database setup and added a resavemodels command.

During createdb sync_translation_fields, update_translation_fields and
resavemodels are executed, giving an out-of-the-box internationalized
project. It is most convenient to configure languages in settings before
running createdb.

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, manually 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 new command is an opt-in, to apply for it add resavemodels_command
to the models class. By default, with modeltranslation enabled, Slugged,
MetaData and Page (and their subclasses) are resaved with the command.

TODO: The post_syncdb infrastructure is risky (the signal is outdated,
      moreover the initial setup should only be done after the inital
      migration). Needs changes on master.
Made modeltranslation fields on lists narrower.

This lets a couple of translations fit. It 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 such 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.
Ignore the registration test complaint about Settings for now.

Placeholder for settings translation.