Rough cut of DrydockRepositoryOperation

Summary:
Ref T182. This doesn't do anything interesting yet and is mostly scaffolding, but here's roughly the workflow. From previous revision, you can configure "Repository Automation" for a repository:

If it's configured, a new "Land Revision" button shows up:

Once you click it you get a big warning dialog that it won't work, and then this shows up at the top of the revision (completely temporary/placeholder UI, some day a nice progress bar or whatever):

If you're lucky, the operation eventually sort of works:

It only runs git show right now, doesn't actually do any writes or anything.

Test Plan:

• Clicked "Land Revision".
• Watched phd debug task.
• Saw it log git show to output.
• Verified operation success in UI (by fiddling URL, no way to get there normally yet).

Reviewers: chad

Reviewed By: chad

Subscribers: revi

Maniphest Tasks: T182

Differential Revision: https://secure.phabricator.com/D14266