Embed STL

Screenshots

Adds STL as a media type for uploads. Adds STL as a filter in media browser for picking items. Provides editor block for embedding a viewer, based on viewstl plugin. Editor block uses media browser to pick model. Options to set model color, background color, border, auto rotation, etc. Editor block updates with changes to match what live site would display. Relevant elements are prefixed to allow displaying multiple viewers on one site page.