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)
deleteSnippetFromStack is recursive and ensures that only one delete is processed at a time (required as each may have to be confirmed)