[Engine V2] init

[michaeljb]

Starting on #12193. No functional changes in the game engine yet.

My in-progress changes can be seen in the branches on my fork starting with "v2" (link).

Before clicking "Create"

• Branch is derived from the latest master
• Add the pins or archive_alpha_games label if this change will break existing games
• Code passes linter with docker compose exec rack rubocop -a
• Tests pass cleanly with docker compose exec rack rake

Implementation Notes

Explanation of Change

• adds Game attr to use as a flag for new changes, @use_engine_v2
• adds scaffolding to create a game via the UI with the flag
  • the option on the create game page to use the flag is only available locally - so for now, any code contributors will be able to local testing of Engine V2, but it will not be available to players of the public site
• the URL query param v2 can be used to override a game's saved Engine V2 setting

Screenshots

"Use Engine V2" checkbox when creating new game:

Game card: