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