sc2: Adding an option to enable war council nerfs

[MatthewMarinets]

What is this fixing or adding?

Adding an option to enable war council nerfs, to hopefully facilitate testing out war council nerf items as they're implemented.
Pairs with Data PR #157
Progress on issue #152

Migration plan

• This setup should work fine for development, but I wouldn't want to ship this unless all the war council nerfs are done. I think core recently added a visibility setting to options, we could use that to hide it if it comes to that?
• We should consider if we want this toggle to be an option in the final release, or if we should just get people to put "War Council" in their start inventory. I could go either way.

How was this tested?

• Set allow_unit_nerfs to true in a yaml, generated, and verified the option was pulled in with /option unit_nerfs
• Generated with the option set to true, checked the spoiler, verified Zealot whirlwind was in the item pool
• Set the option to true, started a mission, verified Zealots, Stalkers, Sentinels, and Centurions were all in their nerfed/non-war council state
• Set the option to false, started a mission, verified Stalkers had phase reactor and Zealots had whirlwind

If this makes graphical changes, please attach screenshots.

None