Allow displaying more users at once in Differential overview

Summary:
I want to check the work of several people (bootcampers) at once.
This implements the OR filter required for that.
In the next diff, I plan to implement also the AND filter which can be useful too.

Test Plan:
Searched for several people, clicked all filters.
Searched for one person, verified that pretty URL is still created.

Reviewers: epriestley

Reviewed By: epriestley

CC: wez, aran, Korvin

Differential Revision: https://secure.phabricator.com/D4404