LDEV-4438 Add unique constraint (assessment_uid, user_uid, latest)
Add unique constraint (assessment_uid, user_uid, latest) to
tl_laasse10_assessment_result which will apply only in case latest=1. It
was done by making 'latest' field contain only NULL or 1 (as opposite to
the previously used 0 or 1). This way constraint will apply only in
cases when 'latest' is 1.