Block time estimation

[tomjeatt]

We have a const in the UI which estimates block times as 12s. Real block times are longer, however, which means that functionality that relies on this estimate (e.g. redeem status and vault redeem status—see https://discord.com/channels/745259537707040778/920489659790929960/1151545332799373352 and #1554) is not working.

We could adjust block times, but that would still be inaccurate. The task here is to investigate the alternatives, and implement whichever gives us the most accurate expiry time for redeem requests (and other related functionality).