message: Make the room-level URL preview notification explicit
  Fixes
diff: @@ -64,7 +64,7 @@ module.exports = React.createClass({
  );
  } else {
  let str = _td("URL previews are enabled by default for participants in this room.");
  - if (!SettingsStore.getValueAt(SettingLevel.ROOM, "urlPreviewsEnabled")) {
  + if (!SettingsStore.getValueAt(SettingLevel.ROOM, "urlPreviewsEnabled", roomId, /*explicit=*/true)) {
  str = _td("URL previews are disabled by default for participants in this room.");
  }
  previewsForRoom = (<label>{ _t(str) }</label>);
  