Add Rollback mechanism for the Hashed Storage migration

There are rake tasks for migrating to Hashed Storage, but none for rollback. So customers may rightly worry about getting themselves into trouble.

• Add the inverse of rake gitlab:storage:migrate_to_hashed, including accepting an ID range
• Consider that migrate jobs may still be scheduled (https://gitlab.com/gitlab-org/gitlab-ce/issues/58739)

There is also a reference to EXPERIMENTAL that needs to be changed

• admin/application_settings/_repository_storage.html.haml (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24402)
• improve read_only behavior for projects being migrated/rolledback (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24128)

Documentation

• Will be implemented in https://gitlab.com/gitlab-org/gitlab-ce/issues/58738