Ensure evenly divisible thresholds are adjusted

[jacogr]

Closes https://github.com/polkadot-js/apps/issues/6828 Closes https://github.com/polkadot-js/apps/pull/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