Add Next and Previous UI to PhamePostView

Summary: Creates a new next/previous UI for PhamePosts, and adds a setFoot to PHUIDocumentViewPro for future use in other apps.

Test Plan:
Test first, next, last posts on Phame in mobile, desktop, and tablet breakpoints.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D14919