Assignable tickets

Adds one DB column: Ticket.assigned_id
```sql
ALTER TABLE ticket ADD assigned_id INTEGER;
ALTER TABLE ticket ADD FOREIGN KEY (assigned_id) REFERENCES user(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. (_default_: ```False```)