Adjust AC appeal vote threshold based on participation

[frivoal]

(Split from #882).

To avoid making momentous decisions based on a slim majority in vote with low participation, the W3C's bylaws have provisions for what they call "Requisite Member Vote" where decisions are confirmed by :

• a 75% majority when participation is at or below 15%
• a 2/3rds majority when participation is between 15% and 20%
• a 50% majority when participation is at or above 20%

I think it would be reasonable to adopt the same concept for AC appeals, which currently can overturn decisions via a 50% majority without quorum or any other consideration for the level of participation.

[frivoal]

This would look something like this: 7bc5058

[frivoal]

From the AB:

RESOLUTION: Adopt supermajority requirements for low-quorum AC Appeals using same ramp as Bylaws; add minimum quorum of 5%.