Add support for live tracking of spotter cars and loader cars/trailers

[PunditSharp]

Allow users to opt-in to having their current location shared with other app users.

Options:

• Users can set their own username to display when they opt-in to share their location.
• A list of opted-in users is provided to all users, and users can choose which user(s) they wish to follow/see

OR

• Teams of spotters/loaders are set up, and members of a team can only see the other vehicles in their team (better for privacy, but it's opt-in anyway).