Example geofilter

This is an example geofilter using the existing interfaces. What is striking to me is how much boilerplate code is necessary. I have some ideas how this could be improved.

• use project templates to generate the boilerplate automatically
• store the result in AttributeSet.data instead of a custom model. This is a JsonField, so we can add more data without migrations. On the other hand, I am not sure if I want to encourage filter authors to meddle with AttributeSet directly.
• Do not use custom filters at all and instead provide an interface to import multipolygons as geofilters (inside polygon is a match)