Create a shadow voting system

This PR introduces a "shadow voting" system.
This means adding a song to the playlist as guest costs a vote, without actually voting on the song and thereby changing it's index in the playlist

Todo:
- [x] Remove the playlist timeout, since we're moving to the new system
- [x] Rename the voting system to "guest system"
- [x] The guest system should only be able to be activated if there's a password set. Otherwise everyone has administrator rights anyway.
- [x] Warn the user if there's a blank password set, since the administrator access won't work otherwise.
- [x] Fix all the tests
- [x] Reflect these changes in the mobile API
- [ ] Write five million more tests