Add 'viewer' to some Remarkup callsites

Summary:
I want to implement a {P123} rule to embed pastes, but we need viewers everywhere before it will work with privacy.

This is not exhaustive; many Remarkup callsites haven't been converted to PhabricatorMarkupInterface yet.

Test Plan: Looked at Maniphest, Differential, Diffusion, Phriction; added markup, made edits and hit previews.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D3428