Allow deleting checkpoints

Problem/Motivation

We currently do not allow deleting checkpoints because that would create problems due to a bug in core.
🐛 Deleting a config checkpoint breaks the checkpoint storage Active

Steps to reproduce

Delete the checkpoint by going to the form via the checkpoint id.
Checkpoints become completely broken.

Proposed resolution

fix core
allow delete operation

Remaining tasks

decide if we want to allow deleting all in the meantime.

User interface changes

delete operation becomes available in UI

API changes

none

Data model changes

none