Snippet delete reference

This PR will ask the user for (additional) confirmation before removing snippets which are referenced by pages.

- In the snippet form
- In the snippet list
   - Each item is processed individually and the user will be asked to confirm *for each* snippet which is referenced.

__TODO__

- [x] PHP tests
- [ ] Dialogue should display the name of the snippet for which confirmation is required (necessary for the list view)
We should be. But it is out of scope for this PR. We should also be doing this in the ContentBundle and maybe other places.