Hashed Storage: Document Rollback mechanism

Documentation

• Add basic instructions for the new rollback task in https://docs.gitlab.com/ce/administration/raketasks/storage.html
• Expand on https://docs.gitlab.com/ce/administration/repository_storage_types.html#how-to-migrate-to-hashed-storage with a step-by-step process (i.e. enable for new projects; validate; migrate a small existing batch; validate; migrate all). Instead of just linking to the basic rake task instructions.
• Rewrite the Rollback section in https://docs.gitlab.com/ce/administration/repository_storage_types.html#rollback