Ensure evenly divisible thresholds are adjusted

[jacogr]

Closes #6828
Closes #4588

The correct solution is probably to carry the > and >= but it certainly makes things a lot more complex. In this implementation we actually DRY things a lot. So could be re-visited possibly if really needed. (The hook in the linked PR could be interesting, instead of useMemo everywhere.

[h4x3rotab]

A quick fix in #9419. Instead of using ceil, it should be floor() + 1.

[h4x3rotab]

If you want to find a chain with even council seats, please try Phala (it has 8 members):

wss://phala.api.onfinality.io/public-ws

[h4x3rotab]

The function is identical to:

function calcThreshold (members: unknown[], threshold: number): BN {
  return new BN(
    Math.min(
      members.length,
      Math.floor(members.length * threshold) + 1
    )
  );
}

Tested with:

for (let len = 0; len < 10; len++) {
  for (let threshold = 0; threshold <= 1; threshold += 0.1) {
    // assert left(members, threshold) == right(members, threshold)
  }
}