Settings: Experimental UI flag

[jkppr]

Is your feature request related to a problem? Please describe.
Many front-end features are developed in multiple iterations until they are in their final step. But even during this development cycle some half-baked features are already shipped to the main branch and will end up in a release version.

Describe the solution you'd like
With the introduction of user settings in PR #3048 we can introduce an "experimental UI flag" that is false by default and new UI features would check for this flag and only be visible when it is set true by the user. So users can actively choose to enable new features in the UI to experiment with them.

Describe alternatives you've considered
Working with different development branches. However, this would need to restructure our development workflow and we would get less feedback on new features during the development process.