Take into account container repository migration status when calculating size

Problem

Found in #350919 (comment 913137663).

The size of migrated container repositories is not being displayed in the UI/API as would be expected. This is because in the model here we're returning early unless the repository was created after the Phase 1 start.

Solution

Now that we're doing Phase 2 we must update the logic to also take into account the migration_state of the ContainerRepository instances. If this is import_done, then we should proceed with the size calculation.