Update Bootstrap to modern version

Currently the GUI is using Bootstrap v2.2.2, which is nearing 10 years of age.

I don't think it should take too much to migrate to Bootstrap 4/5, but some things will inevitably be incompatible between versions, so it will take a little bit of tweaking.

Seems Bootstrap 5 can break jQuery, so we should probably stick to 4 unless a larger amount of time can be allocated to this issue to also replace jQuery (although switching to 5 would definitely be preferred as jQuery is also a bit outdated compared to newer JS features and frameworks)