R1:4eab3c4c8d2b

Reindex dashboards and panels (allow migrations to queue a job to queue other indexing jobs)

Summary:
Depends on D20411. Ref T13272. Dashboards and panels have new indexes (Ferret and usage edges) that need a rebuild.

For large datasets like commits we have the "activity" flow in T11932, but realistically these rebuilds won't take more than a few minutes on any realistic install so we should be able to just queue them up as migrations.

Let migrations insert a job to basically run `bin/search index…