Check in at location

[LGro]

Add support for checking in at the current GPS location; with auto check-out when leaving a (predefined) radius. Detail the UX here first.

• [x] Check-in should pick up the current location by default
  • [ ] offer manual location picking as a fallback (we support gps check in without google play services and one can always schedule a manually picked location and check in then in case needed; therefore this is low priority)
• [ ] A time threshold can be chosen to automatically check-out.
• [ ] A radius around the check-in location can be chosen to automatically check-out when leaving it (requires location access).
• [x] Check-out can be triggered manually whenever checked-in.
• [x] Checking in to multiple locations at the same time is not possible.
• [x] When a previously defined future location is reached in time, check-in there should be one tap.
• [ ] When someone else checks-in close to my location, bring it to my attention.