Use auth sources by default by identifying them as "default" in the config. (+3 squashed commits)

Squashed commits:
[090679e6b] authsources must be an array
[1778d0c21] only set the AuthSource if it also exists, but always remove the config
variable.
[c09669e32] add the possibility to specify one AuthSource to be shown and the rest
are used via multiauth:MultiAuth