[Android, iOS] Send push notifications to out-of-date android app users starting with version 3.6.0`

[biblicabeebli]

After implementing #88 we now have the ability to send an arbitrary notification to the app, along with the app version reported by the server.

• This is an entirely backend feature, but it is mostly for Android, though iOS should also probably have this.
• See implement "please update your app" notifications for iOS and android beiwe-backend#390

[biblicabeebli]

Based on the existence of error reports of devices running literally years out of date versions of the ios app, we would want this on both platforms. Updating issue name.