Prune exports with a service

What does this MR do and why?

When a group or project is deleted, database-level ON DELETE CASCADE then removes all records in the hierarchy down to and including BulkImports::ExportUpload.

Bypassing Rails here has an unintended consequence:

• There is no FK relation between ExportUpload, so Upload records are orphaned.
• Because ExportUpload are deleted at the DB level, the associated files never get deleted.

This MR ensures that Rails callbacks are executed when the project or group is removed.

References

#542228

How to set up and validate locally

1. Count the number of existing records: Upload.count & BulkImports::ExportUpload.count
2. Create a group with a project in it.
3. Export the group into a new destination using Direct Transfer.
4. Count the records again and observe the resulting files in object storage.
5. Delete the group.
6. Re-count the records and observe that the files should now be removed from object storage.

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