Firebase Auth - Cordova - iOS - signInWithRedirect - about:blank

### [REQUIRED] Describe your environment

* Operating System version: iOS
* Browser version: iOS Cordova
* Firebase SDK version: 5
* Firebase Product: auth

### [REQUIRED] Describe the problem

The Firebase / Cordova intergration instructions are now missing a step and without it signInWithRedirect will not redirect back to the app and instead show a blank page with the url about:blank.

This issue arises because the app goes to sleep when the browsertab is opened and it never receives the callback once Facebook / Google / Twitter has authenticated the credentials.

This can be solved by installing the Cordova plugin cordova-plugin-background-mode

The sign if flow should now enable background mode before opening the browsertab and disable it after the process has completed.

```
window.cordova.plugins.backgroundMode.enable();

const auth = await this.get( 'firebaseService' ).auth();

var result = await auth.signInWithRedirect( provider ).then(function() { return auth.getRedirectResult(); }).catch(function( error ) { return self.handleError( error ); });

window.cordova.plugins.backgroundMode.disable();
```

The firebaseService is an Ember intergration and can be replaced with the standard Firebase way.