Zurich - make problem rejected email optional

Adds a checkbox when a problem is marked as hidden to prevent the "automatic reply" from occurring.

![screen shot 2013-09-11 at 17 11 25](https://f.cloud.github.com/assets/22996/1124346/e793b4d8-1afc-11e3-947b-d5fb81992b5a.png)

Still need to:

- [ ] Write a test to verify no email is sent when checkbox is not checked

Closes mysociety/FixMyStreet-Commercial#410