* Wrong exception class: ActiveRecord::RecordNotUnique, not PG::UniqueViolation

It's completely not obvious but PG::UniqueViolation is just a string inside the exception message, not the actual class of the exception

* Favourite does not have target_account_id

* Improve account index migration

- Display more progress in stdout
- Catch PG::UniqueViolation when re-attributing favourites
- Skip callbacks and validations when re-attributing other relationships

* Use in_batches to reduce table lock-up during account merge

* Use #say_with_time to benchmark each deduplication

Under rare circumstances the user record could have already been deleted before...

Fix #6937
Fix #6837
Fix #6667