Display existing reactions below the message

This displays the existing reactions a message has from all users below the
message.

Since we don't currently have an API to actually get these events yet,
adds a temporary hook that looks for a specific message to inject some sample
data. This helps build out the UI for now and can be removed once it exists.

Fixes https://github.com/vector-im/riot-web/issues/9573

src/components/views/messages/ReactionsRowButton.js

@@ -0,0 +1,33 @@
+/*
+Copyright 2019 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import React from 'react';
+import PropTypes from 'prop-types';
+
+export default class ReactionsRowButton extends React.PureComponent {
+    static propTypes = {
+        content: PropTypes.string.isRequired,
+        count: PropTypes.number.isRequired,
+    }
+
+    render() {
+        const { content, count } = this.props;
+
+        return <span className="mx_ReactionsRowButton">
+            {content} {count}
+        </span>;
+    }
+}