Add dynamic_user_statedir configuration option.

[bcbnz]

This allows setting the StateDirectory option for a dynamic user to a subdirectory of /var/lib, allowing for easy backups or persistence between containers. By setting c.SystemdSpawner.dynamic_user_statedir = "jupyterhub_users/{USERNAME}" all state directories can be easily backed up from the /var/lib/private/jupyterhub_users directory instead of having to find the individual usernames.

The default is "{USERNAME}" which matches the previously-used state directories.

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