[MM-33828] Define CSP for all BrowserWindows, enable contextIsolation for main/settings window, other fixes (#1517)

* [MM-33828] Added CSP to renderer process pages and turned off remote module when not needed

* Turn on contextIsolation and turn off nodeIntegration for main and settings windows

* Check for details.reason instead of object

webpack.config.main.js

@@ -17,6 +17,7 @@ const base = require('./webpack.config.base');
 module.exports = merge(base, {
     entry: {
         index: './src/main/main.js',
+        mainWindow: './src/main/preload/mainWindow.js',
         preload: './src/main/preload/mattermost.js',
         modalPreload: './src/main/preload/modalPreload.js',
         finderPreload: './src/main/preload/finderPreload.js',