Lift bulk tests for "many users against one object" capabilities into "PolicyFilterSet"

Summary:
Ref T13602. Currently, the policy framework can not execute "test if many users can see one object" particluarly efficiently. This test must be executed more broadly to implement the changes in T13602.

To avoid making this any worse than it already is, lift this block into a wrapper class that has a bulk queue + fetch API and could eventually be optimized.

Test Plan: Viewed a task with an @mention of a user without permission to see it in the summary, saw it rendered in a disabled style.

Maniphest Tasks: T13602

Differential Revision: https://secure.phabricator.com/D21546