Prevent media from autoplaying when rendered as a feed story

Summary: Fixes T11845. Users can still embed a text panel on the home page to give it some ambiance.

Test Plan: Wrote an autoplay video as a comment, saw it in feed. Before change: autoplay. After change: no auto play. On task: still autoplay.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11845

Differential Revision: https://secure.phabricator.com/D16920