Alert if the app is not active in the background

[bjorkert]

Shows an alert if the app hasn't been active for 6 minutes in background mode. BG is checked at least every 5 minutes.

[marionbarker]

Summary

Success, merging to dev

Testing

Build with this branch

• make sure data is coming in from data source
• return to home screen
• quit the app, lock the phone
• wait at least 10 minutes in each case
• with Notifications enabled (and persistant) for LoopFollow (in iOS phone settings)
  • Notification is visible warning that LoopFollow is not running (graphic below)
• with Notifications disabled for LoopFollow (in iOS phone settings)
  • nothing happens
• enable Notifications; must open LoopFollow one time, then can repeat the test and get the alert (phone locked or unlocked)