Make closed tasks appear grey

Summary: In long list of tasks, its hard to differentiate between open and closed tasks. This adds some color in addition to the strikethrough. Hover will still be blue. I'm open to other suggestions as well if you don't think this is needed.

Test Plan: Test Unbeta Pholio

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5270