Flags - add ability to group by color

Summary:
I use color to convey meaning like "good resource to keep handy for a bit on new way of doing things" or "snipe this task". Now the list can be grouped by these colors.

Note I do this in PHP 'cuz color isn't part of any index AFAIK and pragmatically speaking this dataset should be tiny in the context of "user flags".

Ref T1809

Test Plan: selected group by color and observed the flags were indeed grouped by color

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, aran

Maniphest Tasks: T1809

Differential Revision: https://secure.phabricator.com/D7188