SNAP emergency allotment dates from snapscreener.com

[snapscreener.com](snapscreener.com) has a useful state-level table with eligibility parameters that I used to add the 50-state SNAP parameters in #734. However, I wasn't sure how to interpret the COVID emergency allotment column (some `null` values were present in both states which have expired the EA and states which haven't), so I used the [linked USDA source](https://www.fns.usda.gov/snap/covid-19-emergency-allotments-guidance) to find the time period each state had the EA active for. Is there a better way to use the EA column for this?

cc @benmolin, @MaxGhenis