R1:029cfcfc1939

Add subscriber/mention support to Pholio

Summary:
- Show subscribers.
- When a user is mentioned in the description or a comment, subscribe them explicitly.

Test Plan: {F22370}

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2097

Differential Revision: https://secure.phabricator.com/D3838