ui config

[oliver-sanders]

At present the UI configuration (e.g. font size) is stored in browser local storage.

This is ok, however, not great, especially for accessibility.

In the long run we should consider storing user configuration on the filesystem so that is is more persistent and can move between browsers.

Suggestion:

• Create a user configuration file in ~/.cylc/uiserver.
• For hubless use cases (token authenticated) servers, the UIS can serve this file up itself.
• For hub use cases, the Hub can provide this via a simple JupyterHub service.