Add "invisible" styling/config to Phabricator

Summary: Ref T3322. Depends on D6297. Here are some Phabricator tweaks to complment D6297.

Test Plan: {F47522}

Reviewers: garoevans

Reviewed By: garoevans

CC: aran, chad

Maniphest Tasks: T3322

Differential Revision: https://secure.phabricator.com/D6298