Allow setting custom product thumbnails

## Problem

Printful seems pretty nondeterministic in what thumbnails it sets, and we don't get multiple thumbnails on the index. This means we can't pick which color is the default, and we can't show multiple sides of products like hoodies or pillows.

## Proposal

We can upload custom thumbnails for product by product ID, e.g. `230792069.png` for the Terminal pillow. If this image exists, the store will use it on the index, and if not, it will fall back to what Printful gives us. This will also allow us to do more creative thumbnails, e.g. for seasonal promotions.