Custom sides for modular paint stack

This allows for the creation of custom sides that work with the Modular Paint Stack (MPS). Sides can be anywhere from 30mm tall to the maximum z-height of your printer. For hanging paints, prefer brackets on the top. For standing paints, prefer brackets on the bottom. You can offset the bottom brackets from the bottom in millimeters. Since these can get rather tall and skinny, a brim is generally a good idea. Smaller sizes can print fine without a brim but larger sizes risk tipping over. See https://github.com/IanGClifton/modular-paint-stack for more information about the Modular Paint Stack system.