Completely implement cursor paging in Maniphest

Summary: Ref T2625. Fixes user and project paging. Adds visibility-aware project group filtering.

Test Plan: Set page size very small and paged forward and backward in Maniphest, particularly with "Assigned" and "Project" group-by filters.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2625

Differential Revision: https://secure.phabricator.com/D6973