Annotations: change order of sanity checks in cleanup migration

In case of failure, this gives a bit more information.
Annotations: add more detail to sanity checks in cleanup migration
Annotations: add missing project constraints to some queries

Two queries where missing a project constraint and could therefore return not
only objects from the project currently looked at. This could have resulted in
failing sanity checks, while everything was actually okay.
Annotations: use DataMigration class for data migrations

Extending DataMigration makes sure the migration can not be run as dry_run. A
dry run would make no sense for a data migration.