Implement ApplicationSearch in Flags

Summary:
Ref T1809. Provide ApplicationSearch to Flags and allow the user to select flags by color.

@chad might have some design feedback on my control.

Test Plan: {F54131}

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T1809

Differential Revision: https://secure.phabricator.com/D6747