Show city districts

[joergreichert]

Current state: No city districts are shown.

Motivation: Leipzig is quite large, displaying the city district names and polygons may help users in orientation.

Future state: City districts are shown like here: https://joergreichert.github.io/gdk-opendata-plot/

[joergreichert]

Comment in App working group meetup: would too distractive in especially mobile app, people don't relate that much to the (official district), are more related to their neighborhood -> won't fix (for now).