gscan: display multiple user's workflows

[oliver-sanders]

See #635

A desirable feature for users who need to monitor workflows running under multiple user accounts.

This involves crossing user servers which is something we initially rejected and should be carefully considered, however, should be technically possible.

We would display multiple users workflows in the GScan sidebar, however, there would be caveats:

1. If the other users server is not running, we can't list workflows from it.
2. Access would be read only (i.e. we can list workflow statuses, but not perform commands on them).
3. If a user clicks on another user's workflow, it would open in a new tab rather than within the current session.