Elm-ify the push notifications code

[iancanderson]

This removes the JS code that currently backs push notifications in favor of using the Alert and PushNotificationIOS modules that I'm working on contributing to elm-native-ui: https://github.com/ohanhi/elm-native-ui/compare/master...thoughtbot:ia-push-notifications-ios

The behavior should be the same as before:

• If we've never pre-prompted a user about push notifications, show them a pre-prompt dialog
• If they accept, show them the system permissions dialog
• If they accept that, record their device token in AsyncStorage
• If they already have a stop selected:
  • Upsert their installation record on the backend so we can send a push notification to the device in the future
• If they don't have a stop selected, we wait until they select a stop to upsert their installation record