Fix Cannot read property "window" of undefined (#4872)

I have no good way of testing this without enterprise code to build, but with SAML enabled on our build we get an error on line 1 of a js bundle.

```
Cannot read property 'window' of undefined
```

Looking through the bundles that are being loaded, I noticed that this controller looked interesting. And I noticed there was one call to window that did not reference `global` and it was only when SAML was enabled. So it seems to match like a good puzzle piece. 

Happy to get some help debugging this if I'm totally wrong in this.

webapp/components/login/login_controller.jsx

@@ -524,7 +524,7 @@ export default class LoginController extends React.Component {
                 >
                     <span className='icon fa fa-lock fa--margin-top'/>
                     <span>
-                        {window.mm_config.SamlLoginButtonText}
+                        {global.window.mm_config.SamlLoginButtonText}
                     </span>
                 </a>
             );