Feature: Chimera card listing

[onecrayon]

This should be forwards-compatible with Dragonborn, so need to review those spoiled cards to make sure of this format before implementing. But the general idea is to store "enemy" cards alongside player cards, originally just supporting listings but eventually maybe supporting creating custom "decks" or campaigns per #89.

Model changes

New model: Enemy

• id
• entity_id
• name
• stub
• release_id
• enemy_type (e.g. "chimera" vs. "dragonborn")

Need to review Chimera vs. Dragonborn to figure out how to associate behavior, difficulty, threat setups, etc.

Card

• Add column enemy_id as a nullable foreign key to the new Enemy model
• Add generated property is_enemy_card based on whether we are associated with a particular enemy; or make this a boolean field in the database?

Code changes

• Revise all card fetching logic to look for non-enemy cards when working with player cards
• Add flag to the cards listing to be able to return enemy cards
• Add filtration support to card listing to be able to view enemy cards of a specific enemy and/or aspect