add logical not to UACL ops

[dannygoldstein]

This PR adds the functionality to create new UserAccessControls that are the logical NOT of other UserAccessControls.

This is useful for defining access controls that only allow access if a certain condition is NOT met.

The implementation here uses an anti join against the predicate to be negated.

[pep8speaks]

Hello @dannygoldstein! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file app/models.py:

Line 638:80: E501 line too long (82 > 79 characters)

[dannygoldstein]

This is actually not needed for what I am trying to do, so closing.