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}

Reviewers: chad, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2231

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