Delete events without submitter

This will clean up the database.

Fixes https://github.com/openSUSE/osem/issues/329 and https://github.com/openSUSE/osem/issues/324
If we have records in event_users table for a submitter and a speaker, who is a different user, event.users still returns a result even if the record of one of them is gone. 
So event.users.blank? will be false but event.submitter.blank? will be true.

Within the application we specifically call event.submitter, so for the time being we don't just need for event.users to be valid, we need for event.submitter to specifically be there and for at least one event speaker to exist too.