[1850] Fix price protection

[vandamm]

Before clicking "Create"

• Branch is derived from the latest master
• Add the pins 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

Per 1850 rules a president can use price protection even if he already sold shares in the company.

This is different from 1870 rules, so this fix is needed.

Any Assumptions / Hacks

• I've decided to not allow president to use price protection to buy the shares he sold himself, otherwise it leads to possible infinite SRs.
• Migrate script was able to repair the fixture, so most possibly pins won't be needed, only migration of existing games as they are easily fixable by adding a pass.