Add a "disabled" style

Summary: Fixes T3525. This feels way better, although it's still a little hard for me to pick out of lists with otherwise default-colored items.

Test Plan: {F49910} {F49911}

Reviewers: chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T3525

Differential Revision: https://secure.phabricator.com/D6435