Ticket transfers do not allow new user to mark ticket as "available"

[stephenyeargin]

Related: #121

The lack of being able to handle ticket state changes has manifested itself in a fairly common scenario.

• User A has tickets that they cannot use
• User B offers to take tickets to either sell on a third party site or to offer around their workplace
• User A transfers the ticket(s) by assigning them to that user
• User B receives tickets, but cannot mark them as still available on SeatShare (doing so will revert them back to User A as unassigned)

Backdoor solution involves changing the user_id (ticket purchaser) on the ticket from User A to User B. Doing that will break the chain of custody of the ticket (User B did not purchase the tickets, but now has leeway to do whatever with them).

/cc @bval