add JumpToBottomButtom component

src/components/views/rooms/JumpToBottomButton.js

@@ -0,0 +1,32 @@
+/*
+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 { _t } from '../../../languageHandler';
+import AccessibleButton from '../elements/AccessibleButton';
+
+export default (props) => {
+    let badge;
+    if (props.numUnreadMessages) {
+        badge = <div className="mx_JumpToBottomButton_badge">{props.numUnreadMessages}</div>
+    }
+    return (<div className="mx_JumpToBottomButton">
+        <AccessibleButton className="mx_JumpToBottomButton_scrollDown"
+            title={_t("Scroll to bottom of page")}
+            onClick={props.onScrollToBottomClick}>
+        </AccessibleButton>
+        { badge }
+    </div>);
+};