Give disabled crumb actions a distinct visual style
Summary: Ref T603. When you aren't allowed to take a top-level action (usually "Create Thing"), visually disable the action.
Test Plan: {F69596}
Reviewers: chad
Reviewed By: chad
CC: chad, aran, asherkin
Maniphest Tasks: T603
Differential Revision: https://secure.phabricator.com/D7318