Support custom model images

[Floppy]

We could let people upload images of their models for the rosters. Might be able to use browser-local storage to remember them between runs, based on the model IDs from the roster?

https://www.reddit.com/r/killteam/comments/pu4n87/comment/he2ed4e/?utm_source=reddit&utm_medium=web2x&context=3