Provide "harbormaster.buildplan.edit" in the API

Summary: Depends on D20217. Ref T13258. Mostly for completeness. You can't edit build steps so this may not be terribly useful, but you can do bulk policy edits or whatever?

Test Plan: Edited a build plan via API.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13258

Differential Revision: https://secure.phabricator.com/D20218