R1:0ce4f6d17687

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