Stone base for 28mm stargate

Remixception! A remix of Pfelpensson's remixed stargate, originally by wtgibson. This is the updated base only. For the actual gate, just use Pfelpensson's stargate.

I decided to make the base for this model a little more interesting, so I did some light sculpting and added stonework to it.

The model was tested and sliced in the latest version of Cura as of 02/08/18.