[#2623] Separate ability score locking from fixed improvements

[arbron]

Previously if you granted an ability score a fixed improvement it also locked that score so it cannot be improved further. This moves the list of locked ability scores into a separate list so scores without fixed improvements can be locked, and scores with fixed improvements can still be improved further.

This is a slightly breaking change because fixed improvements that were previously locked will no longer be so.

Closes #2623