Add SEO keywords/description/title support to the `BlogPage` root.
Port to django-fluent-utils, for better code sharing.

* Soft dependencies to django-taggit and django-fluent-pages are more clearly defined now.
* Introduced CurrentPageMixin, which exposes the `page` for blog views.
* Replaced Django 1.3 compat imports with their real Django imports
Add fluent_blogs_archive_index template for the index view.

This makes it possible to give it a separate look compared to the
_archive_year, _archive_month views. Also display SEO
keywords/description here if possible!
Apply MultiSiteAdminMixin for multisite support