Additional zero bid logic for Society pallet

[shawntabrizi]

This introduces some additional logic around users trying to join society with zero value bids.

1. We will only select one zero value bid to be a candidate per rotation period. This zero bid will be the first of the zero bids that applied for entry into society. The rest of the selected candidates will comprise of users whose bids have some non-zero value.
2. If the accepted candidate pool contains a user with bid value of zero, that user automatically becomes the Head no matter the number of approvals they recieved. Otherwise, a random candidate is selected as head, weighted by approvals (as it was before).

A new test verifies that these two points are handled, correctly, and all other existing tests/logic continue to work unaffected.

[gavofyork]

will the zero bid always be the first in the list? if not then the primary_point bidder will be chosen if it comes before the zero bidder.

[shawntabrizi]

Yes. We must assume bids is sorted by value. If so, then candidates will be sorted by value. If so, then accepted will be sorted by value.

If you would like me to write this assumption down near this code I can do that.

[shawntabrizi]

I will work on another implementation of this logic introducing a new storage item ZeroBids, but if you like this, then feel free to merge it in and I can scrap the other work.

[gavofyork]

i think it would still be worth cleaning it up if you've already started, but can merge this as is since it's strictly better.