Inform users if their CRUD actions result in overlapping absences & the consequences of that

The following discussion from !52 (merged) should be addressed:

• @yuha started a discussion: (+2 comments)

  @hansegucker @ZugBahnHof two questions:

  1. We should probably inform the user in case of overlapping absences and the consequences of creating/editing an absence in a way that leads to overlaps (cuts to existing absences). How exactly should this happen? I'd think of something like the overview step when creating absences in the coursebook that shows which lessons will be affected.
  2. This is more related to Alsijils use of Kolego absences, but how exactly should instances that relate to Kolego absences be updates in case these said absences change?

assigned to @yuha