Add support for settings.RICHTEXT_FILTERS

RICHTEXT_FILTERS is a list of items that are valid for the
RICHTEXT_FILTER setting.  The plural version takes precedence if it is
available and non-empty.  Each item in RICHTEXT_FILTERS is applied in
order.

An alias for the ``richtext_filter`` template filter has been added to
match the plural nature of the new setting: ``richtext_filters`` simply
calls on ``richtext_filter`` for its output.
Fixed blog post categories list in blog_post_list.html template
Removed redundant jQuery media from KeywordsWidget
Use urljoin in Page.get_absolute_url for link pages
RTL: fix position of changelink icon in page tree after recent changes

Problem introduced in aec1a0462b60, which solves an issue due to long
page names.
A few Arabic translations in mezzanine/accounts
update ru translation
Added Tim Valenta to AUTHORS.
Merge.
Merge.
Comma separate categories in blog post listing.
Added Artem Gluvchynsky to AUTHORS.
Merge.
Merge.
Added Dheeraj Sayala to AUTHORS.
Merge.
Merge.
Update docs for new RICHTEXT_FILTERS setting.
Properly deprecate RICHTEXT_FILTER setting in favour of RICHTEXT_FILTERS (plural) setting.
Update templates to use new richtext_filters (plural) tag.