R1:fa273523096f

Rough in EditEngine for Maniphest

Summary:
Ref T9132. I'm going hold this until after the release cut since it isn't going to land completely smoothly, but I think I can prep it today/tomorrow and hopefully get it close enough to working to put in HEAD on Saturday after the push.

This adds the basics: new EditEngine, new EditController, and new `maniphest.edit` API endpoint.

I put the new stuff at `editpro/` for now until it works a little better.

Some notes on stuff this is dropping/changing/not…