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
@wachterjohannes I have added this as since the loadLocalizedUrls feature, the webspace is a required argument.

This is a problem as when we validate the snippets for references currently for pages which reference the snippet, as the pages could come from *any* webspace.

The logic here is that if no webspace is given we will infer the page from the path of the node.