Don't limit how many wait lists a user gets added to

This way users can be in multiple wait lists and will be automatically
added to the first one in a timeslot with an opening.

This won't be an issue as we already check that all restrictions are
obeyed when filling opened spots from wait lists.
Avoid trying to signup already signed up users

Though the current UI makes it hard to do this it is better not to
depend on this to ensure proper behaviour.
Document cases covered by dangling ELSE of complex IF
Be more uniform with language used