[MM-22102] upgrade to electron 7 (#1268)

* fix errors

* upgrade to electron 7

* getsavepath is deprecated

* fix typo

* disable messaging from 'null'

* replace getname to its property

* deprecated zoomlevel

* deprecated setbadgecount

* isdarkmode deprecated

* it's a property, not a function

* fix error message

* fix test

* fix test on linux

scripts/fix_catalina_dark_mode_debug.js

@@ -1,13 +0,0 @@
-// Copyright (c) 2016-present Mattermost, Inc. All Rights Reserved.
-// See LICENSE.txt for license information.
-
-const {exec} = require('child_process');
-
-if (process.platform === 'darwin') {
-  exec('plutil -insert NSRequiresAquaSystemAppearance -bool NO ./node_modules/electron/dist/Electron.app/Contents/Info.plist', (err) => {
-    if (err) {
-      console.error(err);
-    }
-  });
-}
-