refactor: rework calendar and remove axios, bootstrap, and vuex

This is a big one because it’s hard to make these kind of changes
atomic. This changeset reworks the calendar component which is no longer
dependent on bootstrap and vuex.

As a consequence we can remove axios, bootstrap, and vuex because the
calendar was the last remaining part of the code that was still using
them.

refs #127 #140 #261 #279