Added MarkdownImage component (#6774)

* Added MarkdownImage component

* Fixed unit tests

webapp/components/markdown_image.jsx

@@ -0,0 +1,19 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+import React, {PureComponent} from 'react';
+
+import {postListScrollChange} from 'actions/global_actions.jsx';
+
+export default class MarkdownImage extends PureComponent {
+    handleLoad = () => {
+        postListScrollChange();
+    }
+
+    render() {
+        const props = {...this.props};
+        props.onLoad = this.handleLoad;
+
+        return <img {...props}/>;
+    }
+}