[1844] Must sell in blocks

[philcampeau]

Fixes #10824

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• 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

As per screenshot of conversation with Helmut Ohley, players must sell shares in blocks.

I had to delete one of the fixture files, since it had an instance of selling multiple shares as separate actions.

Screenshots

Any Assumptions / Hacks