Make deregister remove all the clients deployments

The changes from #50 (closed) should be integrated with the deregister client call like so:

• Client requests deregister
• during the handling of the request the sites services are deleted (maybe also the site itself?)
• the deletion causes DeploymentStates to be orphaned
• Response should look like the response to a standard fetch by the client
• the client handles this fetched data as usual (i.e. removing the deployments)
• the client shuts down

This needs changes to the deregister view and the clients handling of the response obviously.