Add basic support for assigning tickets.

Adds one DB column: Ticket.assigned_id
Adds one admin-only view: "Tickets assigned to me" (/tickets/assigned)
Adds one config option: SHOW_ASSIGNED: whether or not to show assignment
                        identity to clients, defaults to off/False.
Close unmatched div tag
Merge pull request #38 from Centarra/assignable-tickets

Assignable tickets