Require a viewer for Remarkup rendering

Summary:
Provide a viewer to all remarkup engines.

This fixes commit summaries in Diffusion, which were failing to link because they didn't have a user and thus couldn't see/load D123, e.g.

Test Plan: Grepped for engine creation.

Reviewers: vrana

Reviewed By: vrana

CC: aran, edward

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D5152