DB: change architecture of locations table

[sofiia-tesliuk]

Sort locations by city_id. Don't use field city_id in each location.
This is needed for optimization of GET requests, as we always need locations only from one city.

From

locations: {
    location_id: {
        city_id: int,
        other_location_info ... 
    }
}

To

locations: {
    city_id: {
        location_id: {
            some_location_info ... 
        }
    }
}

[sofiia-tesliuk]

TODO:

• Update architecture in Firebase
• Update functions to manage with new architecture:
  • get_receive_stations()