Legacy storage support: Design repositories should be moved when project is transferred to another namespace

Currently, Projects::TransferService will move just the main project repository and the wiki repository when a project is moved to another namespace. We should also move the design repository.

Like the main project and wiki repositories, the design repository move should be rollbackable.

This was mentioned in #13428 (comment 256200242) and is one part of two remaining issues to have design management support legacy storage (see #13428 (comment 258798095)).