Store SR updatedAfter date in Param Store (or Lambda environment vars)

[mathomas]

It's looking like there's no need for this to be a dynamically-calculated thing. It's probably better to just use it as a date to keep us from digging way into the past for candidates once we go live.

For example, if we go live on 08/01/2018, we might set the date to 07/21/2018 so we'd only reach back that far for candidates.