Add Persona auth provider
Summary: Ref T3958. Adds a provider for Mozilla's Persona auth.
Test Plan:
- Created a Persona provider.
- Registered a new account with Persona.
- Logged in with Persona.
- Linked an account with Persona.
- Dissolved an account link with Persona.
Reviewers: btrahan
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T3958
Differential Revision: https://secure.phabricator.com/D7313