Use an extended policy to bind column and board policies together

Summary:
Ref T10349. Columns have the same policies as the projects they belong to.

However, the current implementation just returns the policy directly. This usually works, but if the project has a policy like "Members of (This) Project", the policy filter tries to check if the viewer is a member of the column itself. That doesn't work, since columns don't have members. This leads to a situation where columns on "Editable By: Project Members" projects can not be edited.

Instead, return a permissive base policy and then use an extended policy to bind the column policy to the project policy.

Test Plan:

• Edited a column on an "Editable By: Members of Project" board.
• Added and ran a unit test covering this case.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10349

Differential Revision: https://secure.phabricator.com/D15268