Rough cut at new "pro" Diffusion edit UI skeleton

Summary:
Ref T4292. This puts a very rough skeleton in place for the new "Manage Repository" UI, somewhat similar to the "Settings" UI.

Right now, it has one panel with no content, and is not reachable from the UI.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4292

Differential Revision: https://secure.phabricator.com/D15683