store user preferences

[oliver-sanders]

I thought we had an issue for this but couldn't find it.

Store user preferences for the UI.

We've discussed this in the past, I think the conclusions were:

• Store user preferences on the filesystem seen by the UI Server.
• We may be able to use JupyterHub to achieve this.
• (Ideally this would be stored in a file which users could SCP to other locations.)

Examples of preferences:

• Job themes (e.g. colour-blind) -#9
• App themes (e.g. dark/light)
• Default view(s)
• View settings (e.g. graph view transposed by default)

Pull requests welcome!