Let players and monsters have images

[mpirnat]

Combatants -- both players and monsters -- should be able to have an image URL associated with them for direct display in a browser or in the player view. This should be added to the various templates as well as existing data, and should be an empty string by default.

It might be nice to also support relative paths on disk in case we want to start bundling monster images with content packages. This would allow offline play without internet access (though that's probably not a huge priority in the age of social distancing, but just in case...).

[mpirnat]

A reasonable first draft of this is now done! Yay!