Combine data-scroll-token and -contained-scroll-tokens

- Instead of using one attribute, use one that might just contain one token
 - Use the first token when tracking a child
 - Mandate that no commas can be in individual tokens

src/components/structures/MessagePanel.js

@@ -472,7 +472,7 @@ module.exports = React.createClass({
         ret.push(
                 <li key={eventId}
                         ref={this._collectEventNode.bind(this, eventId)}
-                        data-scroll-token={scrollToken}>
+                        data-scroll-tokens={scrollToken}>
                     <EventTile mxEvent={mxEv} continuation={continuation}
                         isRedacted={mxEv.isRedacted()}
                         onWidgetLoad={this._onWidgetLoad}