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Apparently it was still unclear to people what call they should use on
registration. Added extra message to explain this further.

Before this commit when a newRef was entered on user registration and
that ref already existed we errored out. Now we use exactly that
reference when it already exists.

Script clears calls, users, references and shadow calls from the
database to clean up after the last contest.

When a user enters a call beginning with DN on the registration page we
now disable the DN-call and rig question fields, so we don't confuse
our SWLs.

Everyone was complaining, everything was on fire!