Allow users to mute a given submission until there is further activity.

Fixes #409. 

On the submission page there is a new mute button on the top-right. When a user mutes a submission, it will no longer show up in the listings in that user's dashboard. 

A submission is unmuted for all users when one of the following happens:
* A new nitpick is added
* An approval is added
* The owner of the submission asks for more opinions

There are no notifications sent when something is unmuted because it is assumed that you had taken part in the discussion, so you will already be notified because of that. 

Let me know if you have any issues with the logic, or anything else. Cheers!