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:

cylc-uiserver/cylc/uiserver/app.py

Lines 454 to 461 in b007b5f

	ioloop.IOLoop.current().add_callback(
	self.workflows_mgr.run
	)
	# configure the scan interval
	ioloop.PeriodicCallback(
	self.workflows_mgr.scan,
	self.scan_interval * 1000
	).start()

Jupyter Server may acquire a less hacky interface for this in the future: jupyter-server/jupyter_server#1329

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