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.

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>
  );