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.

Ran unit tests.