Adapt and clean up old events

Removed all references to Participant and relationships between events and
anything other than spaces (posts and attachments were related to events).

The old events model/controller/views are now space_events, since they will be
used for events inside spaces only. Several things still have to be removed from
these events to clean it up.

Events will only belong to spaces for now, and the application will use as much
as possible from MwebEvents. What will be left in the application will be only
the necessary to adapt the gem to Mconf-Web.

refs #1115