Upgrade to babel 7 (mattermost#579)

* Upgrade to babel 7

* Disable module transform in webpack build

* Bump up versions of Babel deps

* Update Babel 7 configuration file

- Change config file to babel.config.js
- Add class-properties proposal
- Enable loose mode

* Disable module transform by default, is it better?

* Revert "Disable module transform by default, is it better?"

This reverts commit 31eb0f5.

* Fix mocha setup

* Disable loose mode, it breaks test

* Fix webpack config

* Update babel dependencies

* Fix devDeps with exact version notation

* Fix target settings of babel/env preset to match with webapp

* Set debug true if not production build

* Ignore eslint on babel config

babel.config.js

@@ -0,0 +1,26 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+module.exports = {
+    presets: [
+        ['@babel/preset-env', {
+            targets: {
+                chrome: 66,
+                firefox: 60,
+                edge: 42,
+                ie: 11,
+                safari: 12,
+            },
+            debug: process.env.NODE_ENV !== 'production', // eslint-disable-line
+            useBuiltIns: 'usage',
+            shippedProposals: true,
+        }],
+        '@babel/preset-flow',
+    ],
+    plugins: [
+        ['module-resolver', {
+            root: ['./src', '.'],
+        }],
+        '@babel/proposal-class-properties',
+    ],
+};