Update js dependencies (bootstrap and jquery) due to django-bootstrap4 update

This is necessary since django-bootstrap4 expects a bootstrap javascript version that already includes popper
Bump jquery to 3.5.1
Bump bootstrap to 4.6.0
Remove separate popper lib
Adjust paths in settings. This also makes sure that the local jquery version is always used, which was previously not the case for admin views that depended on code.jquery.com
Remove explicit popper loading that was introduced in 2c359090 to mitigate the effects of the update in c0b3478c