Ability to tell the user about system maintenance. Want a banner on workbench2.

A very simple implementation could be a json url on the server that where workbench2 is hosted where an admin can populate a message to be shown to user, e.g. "we are aware of performance issues and are investigating a fix" or "scheduled maintenance on Saturday November 29th 21:00-23:00, Arvados will be unavailable".

The mechanism could be similar to how we tell workbench2 about the location of the api server, e.g. in the nginx config:

    location /config.json {
        return 200 '{"API_HOST":"ce8i5.arvadosapi.com"}';
    }

This content can of course also be loaded by nginx from a file.