This commit replaces the confusing mess of aliased joins used when
filtering private Worklists and Boards out of queries, with a clearer
approach utilising the relationships defined in the SQLAlchemy model.
This approach will be much easier to extend to support adding Teams
to Board and Worklist permissions.
Change-Id: I5a373dc91315a3f29f4442363da19105d4d18bfc
255894392a