R1:ec8b4457271e

If there are no flagged revisions, don't show anything on "Active Revisions"

Test Plan: Looked at /differential/ with and without flags.

Reviewers: nh, btrahan

Reviewed By: nh

CC: aran, epriestley

Maniphest Tasks: T1055

Differential Revision: https://secure.phabricator.com/D2044