application header: allow custom colours for owner/deployment

[oliver-sanders]

Production systems often use red/green deployments with a live and a development system.

Sites are likely to have different Cylc Hub deployments for these two deployments.

It would be helpful for operators if there was a clear indication of which deployment they are looking at to help prevent the potential for embarrassing mixups.

An old idea which (apparently) hasn't been written up (?) was to use the "deployment" field in the application header to convey this information. Users / sites could configure a colour for each deployment, this would be used to control the background [/ foreground?] colour of the deployment box when this deployment is selected:

For accessibility reasons, the site would probably only colour their live deployment / use contrast.

It has been suggested that it would make sense to extend this to the server owner field too (would be useful for differentiating shared accounts).

[MetRonnie]

Another option would be the version chip at the bottom left which we already alter to highlight if we're using the development build. This never scrolls out of view (although the sidebar can always be collapsed).

Quick mock-up:

[hjoliver]

Or both.