Move event to database, support closed events

Move event information from settings.py to database.

Create a middleware to check that an event is active. Return a "no
event" page when no event is detected (allows to have the website always
running).

Leverage the middleware to push event title/subtitle info into templates.
Remove the context processor we used to that end before.

Add support for 'CFP closed' status that removes the session suggestion
button.

Replace the loadtopics command by a loadevent command that lets you
create the event and topics in the database in one shot.

Change-Id: I1233962606a59fe1d353eaad024c8d06c8fcbeef
Reviewed-on: https://review.openstack.org/34001
Approved: Thierry Carrez <thierry@openstack.org>
Reviewed-by: Thierry Carrez <thierry@openstack.org>
Tested-by: Jenkins