move map rendering to independent app

[kohllisti]

Create an independent repository to contain the map rendering functionality.

[iomn]

We tried to do this but it introduced new complexity because of the following reasons: Rendering service is an async tasks, as backend is. We would need to synchronize both but with different identifiers (as the rendering service renders everything as input and does not know maps and so on). Basic work is done in https://github.com/aktionskarten/renderer . It is more a longterm goal.