Modernize Diffusion "change" view

Summary:

• Kicks it out to full width.
• More useful header/crumbs/properties/actions (needs some more work).
• Works for public repositories.
• Fix a bug where the "rX" crumb would lose the branch you're on.

Test Plan: See screenshot.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7063