More advantage mode coverage

[krbz999]

A continuation of #4838.

Modes:

• Add: Adds one source of advantage (1) or disadvantage (-1).
• Upgrade: Makes it impossible to have disadvantage (0, 1). Does nothing.
• Downgrade: Makes it impossible to have advantage (-1, 0). Does nothing.
• Multiply: Does nothing.
• Override: Forces one advantage mode regardless of other sources, downgrade, or upgrades. Does nothing.
• Custom: is custom.

[roth-michael]

I don't know how I feel about what I'm about to suggest, but might it make sense to do a ui warn notification if someone uses upgrade/downgrade/multiply/override for the time being, informing them that Add is the only functional change mode currently? On the one hand, I kind of hate that. On the other, it would definitely ensure that it gets fixed since it'd pop up every time data prep happens.
Probably not worth it, but I dunno. If nothing else maybe a console.warn