Add a basic tooltip showing who reacted

This adds a first attempt at tooltip showing who reacted to a message. It
doesn't limit senders or position the tooltip nicely, but the info is there at
least.

Part of https://github.com/vector-im/riot-web/issues/9722

src/components/views/messages/ReactionsRow.js

@@ -116,8 +116,8 @@ export default class ReactionsRow extends React.PureComponent {
             return <ReactionsRowButton
                 key={content}
                 content={content}
-                count={count}
                 mxEvent={mxEvent}
+                reactionEvents={events}
                 myReactionEvent={myReactionEvent}
             />;
         });