Decouple the Blog Post List from RichTextPages

The blog_post_list.html template currently requires that any `page`
context variable passed to it has a `richtextpage` attribute.

These changes modify the template, making the `richtextpage` attribute
optional. This allows users to create other Page types pointing to the
URL of the Blog Post List.
Fix styling of admin help text. Closes #1013.
Merge
Further decoupling of richtextpage in blog templates.
Added test to check admin sites dropdown appears.
Apply admin title click globally.
Show users section in admin nav highlighted as active for the change password view.