added hub url config to globalcfg

[markgrahamdawson]

Closes cylc/cylc-uiserver#238

Added hub url to global config.
This is used by cylc-ui-server (see cylc/cylc-uiserver#507) for routing users to the hub on execution of
cylc gui

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present). NA
• Tests are included (or explain why tests are not needed). NA - nothing that can be tested
• CHANGES.md entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.NA

[oliver-sanders]

(closed and re-opened to pick up the CI fix, tests should pass now)