Remind the user to launch the app if updates have stopped

[aaronpk]

Sometimes iOS pauses location updates of the app for unknown reasons. There is currently no indication to the user when updates have stopped. We can use scheduled local notifications to fix this.

• When location updates are first turned on, schedule a local notification for 10 minutes into the future
• When the app receives new location data, cancel the scheduled notification and reschedule it for the future
• If the notification runs when the app is quit, the user will see the prompt. Make sure the actions shown will launch the app

(Note: since the app can receive location data every 1 second, we might need to rate limit how often we reschedule the notification)

[aaronpk]

Let's give this a try. For now I'm not adding a setting for it, but if people want I can make this optional behavior.