Admin: coverage edit 1: join and split polygons

Currently the admin tool only allows polygons to be deleted from the map - usually non-passenger routes that are not wanted on the map. Two extra features would be very helpful: - Join polygons: select two (or more?) adjoining polygons, and have them merged into one. This would help remove unnecessary small polygons. - Split polygon: select a single polygon, and mark a point where it should be divided into two. This would help break down some larger polygons, especially those including both passenger and non-passenger routes.