Feature: set log level via settings UI

[R0Wi]

Resolves: #977

Setting the backend log level is now possible via settings UI. The radiogroup also represents the current loglevel, which is set on the server.

The position of this UI element can be discussed. Maybe it's also worth introducing a new settings button for this.

[github-actions]

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[susnux]

Nice feature, thank you!
There are some comments, most ones are about indention. You should use tabs instead of spaces.

[R0Wi]

@susnux thanks for your review and sorry for the delayed response. I applied all your suggestions and rebased on the current master. Would love to see your final review 👍

[susnux]

Thank you for working on this :)
I found two things:

1. Align the default logging level with the one server uses (WARN)
2. Please validate the given value before setting the system config

[R0Wi]

@susnux implemented all your requested changes. Thanks again for the nice feedback. Code would now be ready for final reveiw 😄

[susnux]

Nice, thank you :)

[szaimen]

🎉