update npm dependencies

It seems like we have missed a few releases. (I believe I executed npm update whenever looking for dependencies. I am pretty sure this used to work in the past, but appearantly we should use npm outdated now.)

Relevant changelogs:

• https://blog.getbootstrap.com/2021/01/19/bootstrap-4.6.0/
• https://github.com/fullcalendar/fullcalendar/blob/master/CHANGELOG.md
  • icalendar support could be interesting
• https://github.com/dmester/jdenticon/releases
• https://eslint.org/blog/2020/02/whats-coming-in-eslint-7.0.0