Fix Bug 755460: Rebrand login as Persona.
Bug 755460: Rework front page a bit to make Persona login prettier.
Bug 798439: Add newsletter signup box to mothership sidebar.
Fix Bug 798439: Add tests for newsletter signup and fix small issues.

Along with a few tests for the newsletter signup, this updates
django-browserid to the latest version and removes mock_browserid, which
got ported to the library a while back.