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/ReactionsRow.js

@@ -0,0 +1,65 @@
+/*
+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';
+
+import sdk from '../../../index';
+import { isContentActionable } from '../../../utils/EventUtils';
+
+// TODO: Actually load reactions from the timeline
+// Since we don't yet load reactions, let's inject some dummy data for testing the UI
+// only. The UI assumes these are already sorted into the order we want to present,
+// presumably highest vote first.
+const SAMPLE_REACTIONS = {
+    "👍": 4,
+    "👎": 2,
+    "🙂": 1,
+};
+
+export default class ReactionsRow extends React.PureComponent {
+    static propTypes = {
+        // The event we're displaying reactions for
+        mxEvent: PropTypes.object.isRequired,
+    }
+
+    render() {
+        const { mxEvent } = this.props;
+
+        if (!isContentActionable(mxEvent)) {
+            return null;
+        }
+
+        const content = mxEvent.getContent();
+        // TODO: Remove this once we load real reactions
+        if (!content.body || content.body !== "reactions test") {
+            return null;
+        }
+
+        const ReactionsRowButton = sdk.getComponent('messages.ReactionsRowButton');
+        const items = Object.entries(SAMPLE_REACTIONS).map(([content, count]) => {
+            return <ReactionsRowButton
+                key={content}
+                content={content}
+                count={count}
+            />;
+        });
+
+        return <div className="mx_ReactionsRow">
+            {items}
+        </div>;
+    }
+}