Get notifications working on Android

[KateCourt]

• [x] Save FCM token along with user ID in firestore
• [x] Add code for displaying notifications in app
• [x] Test with using specific FCM tokens from console (while waiting for cloud function to work)
• [x] Test with cloud function
• [x] Redeploy with new google-services.json now have added Google Play signing key to see if that works
• [x] Test with real devices
• [x] Test with real devices uninstalling app to test change in FCM token works
• [x] When user clicks on safety notification take to wherever this option is?

[KateCourt]

When running on emulator and running through my phone through android studio, FCM token saves correctly in Firestore and individual and group notifications work. When running the app from the app store neither works and FCM token not saved to Firestore (you'd expect at least group notifications to work as these don't require FCM tokens)