This migration fixes any PreregistrationUser objects that might have
been already corrupted to have the administrator role by the buggy
original version of migration 0198_preregistrationuser_invited_as.
Since invitations that create new users as administrators are rare, it
is cleaner to just remove the role from all PreregistrationUser
objects than to filter for just those older invitation objects that
could have been corrupted by the original migration.
Tim Abbott