[OFFAPPS-324] Handle creating children of closed tickets

:koala:

Handle creating children of closed tickets

![linked_ticket_closed](https://cloud.githubusercontent.com/assets/153219/4820052/d6e5c638-5f0e-11e4-88ed-cbecddedd19b.gif)

I'm not sure if this is the right UI behaviour though to be honest.

/cc @zendesk/quokka

### References
 - Jira link: https://zendesk.atlassian.net/browse/OFFAPPS-324
 - Zendesk ticket: https://support.zendesk.com/agent/tickets/871469

### Risks
 - None