Migrate app to TypeScript (#1637)

* Initial setup and migrated src/common

* WIP

* WIP

* WIP

* Main module basically finished

* Renderer process migrated

* Added CI step and some fixes

* Fixed remainder of issues and added proper ESLint config

* Fixed a couple issues

* Progress!

* Some more fixes

* Fixed a test

* Fix build step

* PR feedback

src/renderer/components/ExtraBar.tsx

@@ -0,0 +1,49 @@
+// Copyright (c) 2016-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+import React from 'react';
+import {Row, Button} from 'react-bootstrap';
+
+type Props = {
+    darkMode?: boolean;
+    goBack?: () => void;
+    show?: boolean;
+};
+
+export default class ExtraBar extends React.PureComponent<Props> {
+    handleBack = () => {
+        if (this.props.goBack) {
+            this.props.goBack();
+        }
+    }
+    render() {
+        let barClass = 'clear-mode';
+        if (!this.props.show) {
+            barClass = 'hidden';
+        } else if (this.props.darkMode) {
+            barClass = 'dark-mode';
+        }
+
+        return (
+            <Row
+                id={'extra-bar'}
+                className={barClass}
+            >
+                <div
+                    className={'container-fluid'}
+                    onClick={this.handleBack}
+                >
+                    <Button
+                        bsStyle={'link'}
+                        bsSize={'xsmall'}
+                    >
+                        <span className={'backIcon fa fa-1x fa-angle-left'}/>
+                        <span className={'backLabel'}>
+                            {'Back'}
+                        </span>
+                    </Button>
+                </div>
+            </Row>
+        );
+    }
+}