upgrade to bootstrap 5.x
Bootstrap 5.x is out, so let's upgrade to it!
The major benefit from using Bootstrap 5.x is that it no longer depends on jQuery, which significantly reduces the amount of JavaScript that gets downloaded.
A nice side-benefit is also dropping Popper, which technically we could have dropped earlier... I just didn't bother, because it drowns in comparison to jQuery.
One down-side is that we need to implement our own Jumbotron component, as that has been removed. But that turns out to be just a few lines of CSS, so I don't think we're going to miss it. It also gives us some more flexibility if we want to change how it looks, for instance in unifying background colors.
For now, I've opted to upgrade without changing any visual appearance, with two exceptions:
- Bootstrap 5.x supports an xxl breakpoint, which makes the max-size for the main container a bit nicer. This is nice for bigger screens.
- The default color palette in Bootstrap 5.x has been tweaked a bit, leaving us with a slightly darker top-nav. If we want this back, we could probably override it, but I don't really see the point.