add geoJSON input for Priority Bounds on challenge create/edit form

[CollinBeczak]

This pr allows users to make polygons for the task priority bounds feature using json.

[file].json layout:

{
  "features": [
    {
      "type": "Feature", 
      "geometry": { 
        "type": "Polygon", 
        "coordinates": [
          [
            [lng,lat],
            [lng,lat],
            [lng,lat],
            [lng,lat],
            ...
          ]
        ]
      }
    },
   {
      "type": "Feature", 
      "geometry": { 
        "type": "Polygon", 
        "coordinates": [
          [
            [lng,lat],
            [lng,lat],
            [lng,lat],
            ...
          ]
        ]
      }
    },
    ...
  ]
}

[jake-low]

LGTM! Thanks for making those changes.