Support /differential/filter/<filter>/<username>/

Summary: NOTE: I didn't add BC for ?phid=.

Test Plan:
/differential/
/differential/filter/active/
/differential/filter/active/epriestley/
/differential/filter/active/x/ - 404
/differential/filter/revisions/?status=open - search for epriestley
/differential/filter/revisions/epriestley/?status=open
/p/jakubv/

Reviewers: epriestley, nh

Reviewed By: epriestley

CC: aran, epriestley

Maniphest Tasks: T900

Differential Revision: https://secure.phabricator.com/D1797