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

- [ ] 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
New Relic recorded 214 `my_profile_edit` transactions over the last 7 days:
![mdn-nr-my-profile-edit](https://cloud.githubusercontent.com/assets/71928/3555160/1f7a9e7e-0914-11e4-80f6-57570081e5f5.png)