front: vendor bootstrap sncf

[Khoyo]

This dependency has been unmaintained since 2019 (from the last commit before the anouncement). We want to migrate from it, but we should probably vendor it in and cut out what we don't use first.

This also remove a fair number of dependencies, since we're not using any of the bootstrap-sncf js components. Getting to zero security alerts is now an achievable target...

For attribution and license purposes, this comes from https://github.com/SNCFdevelopers/bootstrap-sncf/tree/c810763533302706ff7cab8834761d82603ad25e/src/scss

Note to reviewers: all commits will be squashed before merging, the first one is probably the most interesting one.