Merge new CE remote_mirror migrations into EE, and disable ones that are already in EE
This will hopefully prevent the same kind of issues we were seeing in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5586.
Those issues there were caused by the project_mirror_data table being moved from EE to CE in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18688.
This MR prevents similar issues caused by the remote_mirrors table being moved from EE to CE in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18715.
Both CE MRs had EE counterparts, but those EE counterparts did not include the new migrations being added to CE, so we didn't see these migration issues until the CE-EE merge.