Clean up HoA session lifecycle

Split server-side HoA model concerns into separate subclass,
ServerHoASession.

Implement auto-muting for lobby videos when user joins HoA.

Add "previous videos" control to admin page which lists
titles/thumbnails from youtube api.

Move "Join Hangout-on-Air" control to inside video controls area.

Switch Join hangout-on-air links to use session redirector.  Simplify
the `getSession` handler and update it to recognize HoA's as well.

Add HoA details to /admin/.