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

src/applications/feed/story/PhabricatorFeedStory.php

@@ -462,7 +462,7 @@ abstract class PhabricatorFeedStory
   }
 
   public function newMarkupEngine($field) {
-    return PhabricatorMarkupEngine::getEngine();
+    return PhabricatorMarkupEngine::getEngine('feed');
   }
 
   public function getMarkupText($field) {

src/applications/files/markup/PhabricatorEmbedFileRemarkupRule.php

@@ -252,6 +252,12 @@ final class PhabricatorEmbedFileRemarkupRule
       $autoplay = null;
     }
 
+    // Rendering contexts like feed can disable autoplay.
+    $engine = $this->getEngine();
+    if ($engine->getConfig('autoplay.disable')) {
+      $autoplay = null;
+    }
+
     return $this->newTag(
       $tag,
       array(