[feature] add possibility to add a custom string and show it on the map to identify the correct crowd

[momu]

First of all, thanks a lot for the app.
In the last day I wanted to join a cycle demonstration in Berlin and join them while on the way. That proved to be difficult as there were a lot of bike items (critical maps users) active at the time I checked.

My proposal is to have a short user settable string, which can be set in the app while sharing the own location. This string should be shown on the map when zooming in close enough. By having this people can see more easily if the bike icons/users they are approaching are the right ones.

Wether the string should be reset once the app is restarted/observer mode is toggled is to be discussed.