Allow manual trigger when workflow paused.

[hjoliver]

Close #5963

I've added the "question" label because there's some disagreement on the issue itself:

@oliver-sanders has argued that manual triggering (i.e., with immediate effect) should only be allowed with cylc hold */* (hold all tasks individually) and not cylc pause (pause the workflow).

However:

• we haven't implemented cylc hold */* yet, and it may not be trivial to do it
• the only solution at the moment is "hold after cycle point", which is not intuitive at all
• two primary Cylc sites have highlighted this as a must-have for operational migration - @dwsutherland, @ColemanTom
• this is very easy to implement (+13/-7 code lines without tests) so at the very least it provides a working interim solution
• and finally, I cannot see any problem with allowing manual triggering in a paused workflow - there's nothing wrong with defining workflow pause as pausing all automatic job submission

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).
• [ ] 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.