Merge branch '211-implement-vue-substitution-frontend' of edugit.org:aleksis/official/aleksis-app-chronos into 211-implement-vue-substitution-frontend

Default value is broken.
Default value is id, as was before, does not work.
But object does not work as well, since not the same object.

:value-comparator="(a, b) => a.id === b.id" should work
but selects the wrong default value and makes the field non editable.

=> Bailing for now.

Merge branch 'new-model-based-on-calendar' of edugit.org:aleksis/official/aleksis-app-chronos into new-model-based-on-calendar