Support cleanup of resources for deleted users via the new delete_forever hook in the Spawner base class

Any Spawner working against JupyterHub 1.4.1 and later can now optionally implement use of a `delete_forever` function.

If implemented, the spawner can do cleanup work associated with a JupyterHub admin pressing the Delete button on a specific user. This has been implemented in KubeSpawner for example, see https://github.com/jupyterhub/kubespawner/pull/475. The logic to support such function was added in https://github.com/jupyterhub/jupyterhub/pull/3337.

Perhaps systemdspawner could provide such function, that also is influenced by additional systemdspawner specific options to decide if a user should be deleted, the home directory should be deleted, and perhaps also something else I've missed? I figure we can make this function default to not doing cleanup if we want to avoid a breaking change, which would make this entirely opt-in.

Would this be possible?