[TASK] Cleaned up user check

The user check now uses the new getConfigValue() method to check if it
is active and the code was simplified.

Additionally a new variable $user_logged_in is available, that will be
TRUE if a user is currently logged in.
[FEATURE] User not logged in error

The user will not get a correct error message if he is not logged in.
Before he got an error that he already placed his vote but he was not
even logged in.