Rough sketch of new repository URI editing

Summary:
Ref T10748. Ref T10366. This adds a new EditEngine, EditController, Editor, Query, and Transaction for RepositoryURIs.

None of these really do anything helpful yet, and these URIs are still unused in the actual application.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10366, T10748

Differential Revision: https://secure.phabricator.com/D15815