Allow users to access cylc play --upgrade.

[wxtim]

Final thing to close #429

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).
• Manually 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. No docs
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01% 🎉

Comparison is base (ad4ccc6) 75.42% compared to head (68804b9) 75.44%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #490      +/-   ##
==========================================
+ Coverage   75.42%   75.44%   +0.01%     
==========================================
  Files          12       12              
  Lines        1408     1409       +1     
  Branches      234      234              
==========================================
+ Hits         1062     1063       +1     
  Misses        296      296              
  Partials       50       50              

Files Changed	Coverage Δ
cylc/uiserver/schema.py	71.01% <100.00%> (+0.21%)	⬆️

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[MetRonnie]

Wasn't this already done in #455?

[wxtim]

Wasn't this already done in #455?

Yes. I've become confused because it wasn't synced to Master yet.