Allow people to subscribe by email to feedback threads

[GoogleCodeExporter]

Add an option on the user dashboard to subscribe to feedback threads by email. 
A user who chooses to subscribe would be notified by email of all messages as 
they appear in their dashboard.

Additional possibilities:
 - Add another option in the dashboard to subscribe to all commits to explorations the user is involved with; this would cause them to be notified by email of commits as they are added to their dashboard.
 - Ask the user when they choose their username whether they would like to subscribe to messages by email. This would be desirable as otherwise an option on the dashboard may go unnoticed.

Original issue reported on code.google.com by jacobdav...@gmail.com on 16 Nov 2014 at 12:39