Make PhabricatorProjectQuery extend PhabricatorOffsetPagedQuery

Summary: This class predates PhabricatorQuery; reduce code duplication.

Test Plan: Loaded projects, ran conduit call with offset and limit.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D3180