Store users last map position and last zoom level for next app visit

[joergreichert]

Current state: At every new login the complete out zoomed map of Leipzig is displayed.

Motivation: The user should be able to continue where he/she stopped using the app (the then selected position and zoom level), assuming that a user usually water trees in his/her neighborhood.

Future state: The zoom and position of the map is continuously tracked in backend database and is restored when user logs in again.

[joergreichert]

This feature was very liked in the last working group meeting, so I will add this to the current watering season.