Map sharing implementation

[penguin86]

@tacruc implemented a system to share maps (including favorites) with other users in the nextcloud maps app.

He says:

So the way the Sharing works is actually via the files app. So you create a custom map, all info required for this map is stored in the folder. Therefore sharing the folder gives other users access to the map. The id of the custom map is the folderId of the corresponding folder. So for your bookmap app, we could either add the mapId to the rest API. For sharing the api for files can be used. Or you load the geojson file stroing the favorites via the android files app.

This issue is to implement the sharing feature on this app.