Begin transacitonalizing repository edits and provide a more sensible edit interface

Summary:
Ref T2231, T603. Plan of attack here is pretty much:

  - Built out a new (currently not linked in the UI) edit interface in Diffusion which is transaction-based and has a sensible layout.
  - Build out a new create interface based on PagedForm which dumps into the new edit interface.
  - Throw the old stuff away.
  - Everyone lives happily ever after.

Test Plan:
{F44163}
{F44164}

Reviewers: chad, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2231

Differential Revision: https://secure.phabricator.com/D6029