Minor cleanups to Diffusion breadcrumbs

Summary:

• Remove "Diffusion" crumb (redundant with application icon)
• Put "All Repositories" in its place on the landing page.
• Render the repository crumb as "rX" instead of "X Repository".

Test Plan: Looked at various Diffusion pages.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D4173