fix #1635 enable mixpanel push notifications

fix #1635 
Enable mixpanel push notifications.

To test this PR:

* go to https://mixpanel.com/report/396379/explore 
* filter by your username
* select it
* click "Send a Notification" 
* then select "Push Notification" 
* enter a test message
* click "Continue"
* click "Send this message"

You should receive a notification on your device within 5 seconds. Tapping this notification should open the app (as you tapped the app icon on your launcher).