Enable the importation of Mapbox project drawings

 - Given a Mapbox project layer, Shareabouts' Argo can now draw shapes
   and locations by freehand and our Shareabouts map will mimic these
   shapes in our Shareabouts project.

 - The parser is now implemented for an Argo layer's 'rules'
   attribute to detect imported features like {{feature}}

 - Some caveats:
   - Marker images for point shapes are not imported
   - Only tested on Points and Polygon shapes; Line shapes might not be
     properly imported
   - The 'title', but not the 'description' shows up in the shape's
     clickable popup