This change adds the `migrate-bug-trackers` management command, which runs the bug tracker configuration materialization. By default, it only creates configurations (which is fast and idempotent). With `--backfill-bugs`, it converts stored `bugs_closed` data into bug relations in resumable, ID-ordered batches. Materialization is a true sync, removing links missing from the string, so rows edited through legacy paths re-materialize correctly. The backfill scales with the number of review requests, so it is never run automatically. Following the diff-migration precedent, rb-site upgrade finishes by checking for unconverted bug data and advising the admin to run the backfill at their convenience. Testing Done: Ran unit tests.