Allow users with no CAN_EDIT permissions to silence projects if they want to

Summary: Humble user cannot silence/mute project if he/she has no CAN_EDIT permissions in it. You can actually leave it but if project is locked - then you're scr*wed.

Test Plan:

1. On a testing phabricator instance created a dummy project
2. Changed that project permissions CAN_EDIT to be by admin only
3. Added poor soul with no CAN_EDIT permissions
4. Logged it in with poor soul
5. Tried to silence the project
6. The Project is successfully silenced
7. User is happy :)

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, Pawka

Differential Revision: https://secure.phabricator.com/D20675