Integrate watering data done by the city

[joergreichert]

Currrent state: At the moment we only show the waterings of the LeipzigGiesst users.

Motivation: The companies watering the the trees in behalf of the city don't share their watering amounts at the moment. So it is not clear which trees already got enough water.

Future state: We somehow get the data (e.g. by Excel import or some other integration) and can sum it with waterings of the users, so the water supply is more realistic.