freehand polygons as excerpt area

Created by: hixi

This is just a rough overview of what needs to be done to achieve arbitrary (multi) polygons:

Web Frontend:

• integrate leaflet.draw
  • use custom icon
  • allow freehand polygons to be drawn

Excluding: Conversion Service (geometalab/osmaxx-conversion-service#96):

• take an arbitrary shape (Multipolygon) as cutting-area
• generate a .poly-file 'on the fly'