Ship "Repositories" create button to new Diffusion workflow

Summary: Ref T2231. Get rid of the old create controller and make the button go to the new stuff instead. This will eventually get cleaned up more, but I don't have a clear plan for Arcanist Projects yet.

Test Plan: Clicked button, hit new workflow.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2231

Differential Revision: https://secure.phabricator.com/D7414