[18Uruguay] adds diesel discount.

[philcampeau]

Fixes #11173

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 the code for a $200 discount when trading in a 4, 5, 6, or 7 train for a diesel.

Screenshots

Any Assumptions / Hacks