Scene search and project export should use same bbox handling

[jisantuc]

In #26, we added some handling so that bad bounding boxes are rejected with an error. In #18, we improved how well scene search tolerates different types of objects as bounding boxes. Project export should be just as tolerant of plausible bbox formats as scene search is. Instead of throwing an error for bounding boxes that aren't "x1,y1,x2,y2", it should make an attempt to decode the bounding box from shapely boxes or shapes.