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)
The Hateoas integration should not be the problem here. As long as you don't use any of its annotations the result should be the same. I am not sure about the LanguageController, but the TranslateBundle has to be overviewed anyway. And the TemplateController is not a REST-Resource, so it doesn't make sense to use this functionality there.