check global config for hub url and

[markgrahamdawson]

Closes #238

Uses hub url in global config (see cylc/cylc-flow#5733)
Existing testing needed to work with and without the global config parameter set - monkey patch used to change settings.

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).
• 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

[wxtim]

👍🏼

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.10% 🎉

Comparison is base (b043bb9) 75.31% compared to head (09b47ae) 75.42%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #507      +/-   ##
==========================================
+ Coverage   75.31%   75.42%   +0.10%     
==========================================
  Files          12       12              
  Lines        1422     1428       +6     
  Branches      235      236       +1     
==========================================
+ Hits         1071     1077       +6     
  Misses        301      301              
  Partials       50       50              

Files Changed	Coverage Δ
cylc/uiserver/scripts/gui.py	89.47% <100.00%> (+0.90%)	⬆️

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