Add option to transfer event ownership

It would be nice to as a owner of an event or an admin be able to transfer ownership of an event. For example, if a anonymous user finds a noteworthy event and decides to upload it, and later the organizer wants to manage the event, then an admin should be able to transfer it without delving into the database. Registered users should be able to transfer their events at-will, prompting the receiver to accept or decline the transfer.

At least for a start, having admins be able to transfer ownership or disown events would be nice.