Simplified auth plugin

Simplified version of #1763 

Introducing auth service.
- Stores persona.
- Handles building/destroying Permissions and Auth plugins (moved from AppController)
-Privacy directive uses it.
- Fix #1759 (The replies permissions were not filled, only the reading permissions
because privacy directive used to fill the rest. But a change made it not to touch the permissions of an annotation which has the permissions already set.)

`getInitialUser` isn't used either.