Support mobileconfig signing

They are currently unsigned.

```
https://autodiscover.mydomain.tld/mobileconfig/?email=myemail@mydomain.tld
```

Related to: https://github.com/rseichter/automx2/pull/11

- [ ] Implement https://github.com/rseichter/automx2/pull/11/commits/8206152f153de13a7b8843f9806449256da7bfeb
- [ ] Add tests
- [ ] Add doc