server: migrate to new interface for starting async things

[oliver-sanders]

We need to run some async stuff when we start the server. Currently we are hacking this into initialize_settings:

https://github.com/cylc/cylc-uiserver/blob/b007b5f91491a2be97fa16313dfbbbe57ce7e933/cylc/uiserver/app.py#L454-L461

Jupyter Server may acquire a less hacky interface for this in the future: https://github.com/jupyter-server/jupyter_server/issues/1329

We should migrate to this in due course. Remember to bump the jupyter_server dependency when doing this.