Feature to deactivate the beacon on leader's demand

Similar to issue #4 in this case too, beacon location should be pre-specified; otherwise, it might lead to process failure. Notification scheduler feature of [flutter_local_notifications](https://pub.dev/packages/flutter_local_notifications) can also be used to inform users (leader as well as followers if any) when Hike has started.
As of now, `startsAt` field in beacon model is specified as `DateTime.now` while creating an hike, `createHike` dialog needs to be modified to pre-schedule the beacon.