Require merge requests that include migrations to specify a rollback procedure

In !15137 (merged) a background migration was added, and it was found this migration did not perform well due to the lack of an index. The migration did not offer a way of rolling back the changes, nor was there a rollback procedure defined in the merge request description. While sometimes rolling back is not possible (e.g. when removing data), we need to at least start requiring developers to:

1. Think about how to roll back the data
2. Add a rollback procedure for background migrations (this is already done for normal migrations)
3. Describe this in the merge request, so a release manager or SRE doesn't have to go through the code to figure out how to revert something

Code wise I am thinking of the following changes:

1. We require that background migrations define a revert method
2. We require that migrations that schedule a background migration define a down method. This method should remove any pending jobs its up method` scheduled, then schedule the reverting of migrations
3. BackgroundMigrationWorker#perform needs to take an optional argument that when set triggers a revert, instead of a normal migration
4. We write a RuboCop rule that enforces all this (e.g. by requiring a non-empty revert method in background migration classes)