Add a soft-conclude mechanism

[PeterJCLaw]

There are often cases where an experiment involves giving the user access to something that will expire and seeing how they interact with it. Since that feature or offer has a published expiry, it would be useful to be able to effectively close off an experiment without immediately taking it away from the users for whom it has not yet expired.

A similar use-case which this sort of functionality would also enable is to close off a test once you have enough users in the relevant branches, but then want to see how those users mature over time.

I realise that both of these cases can be handled outside Jacquard, however it would be very useful if Jacquard supported it directly.