Explicitly degrade edge editing for commit/task edges until T4896

Summary:
Commits don't support PhabricatorApplicationTransactionInterface yet, so the "Edit Maniphest Tasks" dialog from the commit UI currently bombs.

Hard-code it to do the correct writes in a low-level way. After T4896 we can remove this and do ApplicationTransaction stuff.

Test Plan: Used the "Edit Maniphest Tasks" UI from Diffusion.

Reviewers: joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D9975