R1:42eaa88f8031

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…