Race for the Galaxy insert

A remix of [Tyrden's original](https://www.thingiverse.com/thing:4214497). I prefer the thicker walls to the 1mm walls of [Patella's](https://www.thingiverse.com/thing:4367253) remix—I find them more aesthetically pleasing as well as more robust.

I think this is a complete list of the modifications I've made on Tyrden's original:

* No assembly required; I've joined it all up as you'd expect for a 3d print model.
* Added a base to the card boxes (0.6mm for a faster print)
* Opened the back of the card boxes (so you can stash behind, like in Patella's version, but with a base so the print is more stable)
* Reduced the number of card slots in the card boxes to 5 from 12 (I don't feel the need to separate the player cards by colour and it makes it stronger I reckon)
* Removed the perimeter so the card boxes are now separate, and made the trays slightly longer to take up the gained space
* Reduced the base on all of the trays from 1mm to 0.6mm (for a faster print)
* Joined the t