Migrate to Bootstrap 5 beta

It is the last version published on their site as "current", we only use a very small part of Bootstrap, but it includes a few upgrades to the grid system, some utility classes, custom CSS variables and more.

https://getbootstrap.com/docs/5.0/getting-started/introduction/

Some differences between v4 and v5:
https://superdevresources.com/bootstrap5-vs-bootstrap4-whats-new/

And a migration guide:
https://getbootstrap.com/docs/5.0/migration/

(TLDR: RTL support, new XXL grid breakpoint, utility classes for paddings, margins, vertical spacing helpers).

Note: they dropped support for IE 10 & 11. But if we are not targeting browsers under 2% of market share, that is fine, IE has around 1%.

This PR should be just about updating Bootstrap and making sure nothing is broken, we'll go into refactors in separate issues. As these things can affect lots of files (e.g.: a search-and-replace through the SCSS files), the sooner we merge small fixes, the better.