Simple "View Only" build with a sketch baked in

[andybak]

The intended workflow here is that the user:

1. Downloads the Open Brush Unity project
2. Adds their sketch into it
3. Hits a "Build" button

Out pops a build for the chosen platform that has their sketch baked-in and automatically runs it in "view only" mode (with the chosen locomotion method). It could be VR or non-VR

We could even run headless Unity on a server and make this a service wrapped in a web gui (does this break Unity TOS?)