R1:ff91884f7d81

Dark sidebar everywhere

Summary:
First pass at testing out a dark sidebar everywhere. Wanting feedback with real test time before implementing before commiting.

Thoughts
- Aligns with Mobile, Tablet UI experience.
- Creates 'application' feel on Desktop.
- Begins to make Phabricator feel like a branded UI.

Cons
- Probably contensious visually.

TODO:
- Update diff view sidebar.
- Make breadcrumbs appear above content area, not above nav.
- Change background texture on crumbs to match table headers.