Ticket #17 - Allow admins to disable certain users' ability to submit tickets.

This adds a simple button in the user invoice admin page to toggle the ability to submit tickets for a user.

Required schema update:

```sql
UPDATE TABLE `user`
ADD COLUMN `can_ticket` BOOLEAN;
UPDATE `user` SET `can_ticket`=1;
```
Or something to that effect.

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To a user whose ticket submitting is disallowed, they should see this on their ticket list.

![cool](http://i.imgur.com/kO65rYe.png)