Design repositories aren't moved when a project repository moves on disk

The DesignManagement::Repository is initialized the same way as a Repository. Meaning it will live next to the project repository with a suffix.

In the case of hashed storage we don't need to do anything, since the paths don't change on disk. But legacy projects need other care:

• If the projects are being migrated to hashed storage in the rename, we need to do the same for the the design repository
• If the projects are not being migrated to hashed storage, we need to rename the repository in Project#rename_repo.

Hashed storage is enabled on GitLab.com.

Proposal

We should make hashed storage a hard requirement of ~"design management". This prevents the need to address this specifically, and done quick enough we won't also need to handle a migration for anyone who uses designs without this requirement.

• The design management documentation should be updated to include this limitation.