Now that bootstrap-vue is no longer used we can finally deactivate the
Vue 2 compatibility mode.

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

This is helpful if multiple operations should use the same error object.

This makes it easier to set styles specifically for the wrapping
container.

refs #247

fixes #207

feat: add gitlab issue templates

See merge request !41

fix: add before_script stage for ci

See merge request !40

Fix docker image names for release

See merge request !39

This reverts commit 4a587f8f.