Add Mozilla Persona Omniauth Gem and initializer
Add (ui-only) Persona button to login modal
Add basic Persona functionality

1. No session integration yet, so automatic login/logout events are suppressed.

2. Popup blockers must be disabled: submits form to target="_blank"
Use AJAX for submitting Persona credentials.

Fixes issue with needing to unblock popups.
Make Persona configurable
Merge branch 'persona-integration' of git://github.com/callahad/discourse
Let's default Persona to off right now, change button text
Check when logging in whether a auth provider is enabled, including specs