build: upgrade bootstrap to v5

Needs rebase and complete refactor.

I.e. the new bootstrap library changes the font drastically. All font-sizes need to be reviewed. Also, spacing of some parts should be reviewed.

@vyskocilpavel - do we still need the update?

restored source branch github/fork/vyskocilpavel/bootstrap

changed target branch from master to main

let's address this in https://gitlab.ics.muni.cz/perun-proxy-aai/python/perun-proxygui (or in the new proxystatistics Python module)

closed