Switch to allauth for Persona, prepare Github integration

This is the next step to enabling Github login. It throws away all of
the django-browserid and custom code and replaces it with allauth's support
for Persona.

Here are the current TODOs for this:

- [x] Fix tests

- [x] Double and triple check migrations for email adresses and social accounts

- [x] Extend documentation

- [x] Update more allauth templates to use our base template blocks and variables

- [x] Open kuma-lib pull request: https://github.com/mozilla/kuma-lib/pull/47

- [x] Look through all allauth provided views

- [x] Update profile Persona display per https://bugzilla.mozilla.org/show_bug.cgi?id=1036134#c4

- [x] Fix site messages in #2598 
@lmorchard - did you do this? and do you remember why? We think it may be related to https://bugzilla.mozilla.org/show_bug.cgi?id=1046568 ... i.e., by removing this line the first request after Persona sign-in may not have the session.