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