Fix c.CylcUIServer.log_level not working in jupyter config

[MetRonnie]

Supersedes and contains one of the fixes from #789

Overriding c.CylcUIServer.log_level in jupyter_config.py was not working.

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).
• Tests are updated
• Changelog entry included
• Docs not needed
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[MetRonnie]

(flaky cylc review test failure unrelated: #807)

[oliver-sanders]

Copying the context from the previous PR...

The issue with this approach is that all logging is supposed to be set by the logging config, this controls everything level, handlers, adapters, streams, etc as an atomic bundle which gets applied in one go. The logging config is a reactive trait which should be able to be edited and reapplied on the fly - modifying the entire setup when it is.

This PR works by hacking the logging (as set up by the logging config) in a way that will break its reactivity, but might corrupt its intention (config depending).

We should really be controlling this via the logging config rather than our application-level. I haven't yet invested the time into working out how that should be done.

Note, the --debug flag sets one value in that config which is all it is intended to do, it isn't really meant to cover user-defined logging config (only default console stream), but I do understand why you think it would be helpful if it did.