Add an option to disable mutual introducer warnings

Feature description

A flag or config option to disable mutual introducer warnings.

Problem or use case

I've been running a cluster with multiple mutual introducers without problems for over ten years, since before the warning existed. I know this means I have to delete devices and folders by editing the config file, and have been doing that all this time, too. Having multiple mutual introducers lets me have a few different machines that can introduce new devices to the whole network, and I want to continue with this setup.

So the warnings are just annoying in this case, and it would be great to be able to turn them off, even if it's with an 'I know what I'm doing'-style flag.

Alternatives or workarounds

Just put up with the warnings. Since switching from syncthing-gtk to syncthingtray as a frontend, this has become less feasible, since syncthingtray sends an OS-level notification every time it gets the warning, whereas syncthing-gtk didn't (there I would only see them on the webpage).

Start by locating where Syncthing detects mutual introducers and emits the warning, then trace how configuration options are defined and tested. Add a narrowly scoped option that suppresses this warning, and verify that the warning remains enabled by default and can be disabled through the supported configuration path.