Remove My Practice Organisation

Takes the organisation out of the practice. The practice members named in keep (user codes) keep access as members of the organisation itself, at their current level; every other practice member loses it. Its own members are untouched. Its HMRC settings stop using the practice's credentials (it needs its own before it can submit), and pending invitations to the practice sent from it are withdrawn. To hand it to the client, transfer ownership too (PUT /tenants/{tenant}/owner). 400 when someone to keep is not a member of the practice; 404 when the practice does not run the organisation; 409 for a practice managed in Sodium.