R1:24a5d0505ae2

Add a "Description" header to the repository view for consistency

Summary: We don't have a section header on `/diffusion/X/` for descriptions right now. Add one to improve consistency.

Test Plan: Looked at a repository.

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D7449