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

- [ ] 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
nit-pick that we don't follow everywhere else: should we use `reverse` here for more future-proofing?