3 no duplicate wblistitems

fixes issue #3.

1) set uniqueness constraint on FilterListItem to (user, url, type)
2) add script to run in staging and prod to remove duplicates from the db
3) add null and blank constraints
4) set URLField max to 500 from 2000 to meet the db-level key max size (set for our uniqueness constraint).

Need to be done:
deploy code to staging and prod.
run script to remove defaults.
run python manage.py migrate api