Feature/song revote

what i did:
anonymous user can not login via login formular
username of an anonymous user is a guid
check against multiple votes also in the backend - post voting route and deny it

state of a voting of a song is set to played when currentSong is set in eventlog
-> so the votes of this song are also removed from the votes list in the frontend and users can vote again on this song

fixed one bug of creating anonymous user when the user is already logged in as anonymous