Replaced unique_together with UniqueConstraint in models that
covered nullable fields as in unique_together database indexes
don't work where subgroup=None. So added conditional unique
index handling invalid duplicate Count data.
Added 0015_clear_duplicate_counts migration to handle existing
data that violates the constraints.
Also corrected a test case in test_counts.py which didn't clear its
state properly and thus was accidentally taking advantage of this
database schema bug.
arpit551