Cut mirroring over to new URIs

Summary:
Ref T10748. This migrates and swaps mirroring to PhabricatorRepositoryURI, obsoleting PhabricatorRepositoryMirror.

This prevents you from editing, adding or disabling mirrors unless you know a secret URI (until the UI cuts over fully), but existing mirroring is not affected.

Test Plan:

• Added a mirroring URI to an old repository.
• Verified it worked with bin/repository mirror.
• Migrated forward.
• Verified it still worked with bin/repository mirror.
• Wow, mirroring: https://github.com/epriestley/locktopia-mirror

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10748

Differential Revision: https://secure.phabricator.com/D15841