Close to real time location sharing

[LGro]

Add a close to real time location sharing feature. Start with outlining the UX; use background location refresh as window of opportunity to send updates as well.

This might just be an additional option in the check-in dialogue to mark e.g. "track live position".

This is the lowest priority one among the location sharing features. Start with check-in and future location sharing first.

[LGro]

There seem to be multiple potentially relevant packages:

• https://pub.dev/packages/flutter_background_geolocation
• https://pub.dev/packages/location
  • "Works either with and without Google Play Services on Android phones" -> couldn't confirm this; was giving me errors about missing GooglePlay
• https://pub.dev/packages/geolocator#listen-to-location-updates
  • this one actually works without GooglePlay services

Generally for background processing:

• https://pub.dev/packages/flutter_background_service