Allow filtering Differential overviews by other participants

Summary:
This is useful for writing feedback - what did I work on with someone?
This creates `AND IN (...)` and not `AND AND AND` if more participants are specified.
User may not expect it but whatever, it works the same for the most common case of 1 extra participant.
It would be nice to have this also for other filters but it would by way harder.

Test Plan: Displayed my revisions with some specific reviewer and some elses revisions with me as reviewer.

Reviewers: epriestley

Reviewed By: epriestley

CC: wez, aran, Korvin

Differential Revision: https://secure.phabricator.com/D4408