Only show reactions in main message timeline

This fixes an error that crashed that notifications panel because it was trying
to read reactions, even though we currently don't aggregate them there. This
change is more explicit about exactly which views should try to show reactions.

Fixes https://github.com/vector-im/riot-web/issues/9713

src/components/structures/MessagePanel.js

@@ -96,6 +96,9 @@ module.exports = React.createClass({
 
         // helper function to access relations for an event
         getRelationsForEvent: PropTypes.func,
+
+        // whether to show reactions for an event
+        showReactions: PropTypes.bool,
     },
 
     componentWillMount: function() {
@@ -541,6 +544,7 @@ module.exports = React.createClass({
                     last={last}
                     isSelectedEvent={highlight}
                     getRelationsForEvent={this.props.getRelationsForEvent}
+                    showReactions={this.props.showReactions}
                 />
             </li>,
         );