849 fix account canceling

fixes #849.

- Requires the *to be deleted* user to be an admin, in order for a `PowerVacuum` to occur
- Refactor non-speaking `2` into `MINIMAL_ADMIN_COUNT = 1`, which is a little bit more readable (and use `<=` instead of `<`.
- Use translations, and elaborate a little bit more on the situation
- Add two specs.