Cleanup, remove render_comment_list from commentsarea templatetags

The upstream django-threadedcomments package ships this tag now,
no need to keep a local copy.
Cleanup/API change: rename placeholder_tags to fluent_contents_tags

Get consistent naming across all packages,
follow coding guidelines and use clear names.
Cleanup/API change: remove support for `{% render_placeholder "slotname" %}`

It was never fully developped in the first place, only worked at
database level. The "Shared content" plugin takes this place now.
Sphinx: allow to link to django:setting:`..` roles.
Support front end media!

Plugins can specify the media to include with the rendered output.
The render() and render_placeholder() methods return a
`ContentItemOutput` object instead of a string now.
However, that object can still be treated as string maintaining
backwards compatibility.

Closes: #24
setup.py: require django-tag-parser 1.0.1 in advance for a bugfix.

The previous version swapped the compile_kwargs/compile_args parameter.
Changelog: mention frontend support too
Add prefix to ContentItem types during syncdb.

Makes them easier to spot in the list of items.
Added `prefix_content_item_types` command for manual updates.
Automatically convert cached output to new format.

Reduce possible 500 errors during upgrades.
Add "Open in a new window" option to picture plugin

The plugin can typically be used for banners, and slidebar labels
hence this feature is typically needed for this element.
Fix escaped HTML comments in rendering output
Fix shared content plugin rendering, it escaped the ContentItemOutput object.
Make sure the ContentItemOutput object can pass mark_safe() unchanged.
Added basic support for multilingual
Integrate django-parler in the sharedcontent plugin.

This makes sure shared content (e.g. a footer) can be translated too.
The "title" is a translated field, so it can be used in templates if
desired.
Sharedcontent: avoid overwriting the slug in new translations

Also make the slug field less prominent,
as it's a developer feature mainly.
Fix stale ContentItem objects when deleting a translation.

Remove the objects manually in the delete_model_translation() hook.
Merge branch 'multilingual_support' into dev
Update translations
Briefly mention multilingual support in the changelog