Split base models for optional translation support

Turns out multilingual support can be made optional,
due to the configuration of base class mixins.
Custom models even remain compatible, as the `AbstractEntryBase` class
maintains it's own old behavior.
Fix managers for base objects
Implement migrations, language settings and admin support

Using new version of django-parler, which has a
add_default_language_settings() function to implement custom language support.
Improve base models + mixins, to be easy to use.

- Added the missing base classes,
- Cleanup top level mixins.
- Add explicit TranslatedField() markers
Add tagging marker `is_translatable_model` to the base models.
Limit detail view URLs to current language only.
Apply 'default_url' pattern to get_absolute_url() too.

Similar to django-fluent-pages@dev
Pass language_code parameter to blog_reverse() and mixed_reverse()

Makes sure the URLs of the blogpage integration are properly resolved to
the correct translated language.
Fix sitemap access

- remove translated 'slug' field.
- only show pages of the current languages
Include real URL in change_form.html

This makes sure the admin URL points to the correct translation.
Hide other languages in the archives and RSS feeds

Using new .active_translations() method of django-parler 0.9.3
Merge branch 'multilingual_support'
Remove unused import
Add missing {{ block.super }} to base_internal.html
Added changelog entries for multilingual support