Add a new admin action "Set default artwork paths based on slug" that
automatically assigns default artwork file paths to UnitGroup objects
based on their slug field. The action sets:
- artwork to "artwork/{slug}.png"
- artwork_thumb_md to "artwork-thumb-md/{slug}.png"
- artwork_thumb_sm to "artwork-thumb-sm/{slug}.png"

This makes it easy to batch update multiple unit groups from the admin
interface without manually editing each one.

Added comprehensive unit tests to verify the action works correctly
with both single and multiple unit groups.