Dates in outbreak events

[noamross]

Some notes on things to be fixed in disease outbreak events (not high priority, more to be added as I dig into this).

• Dates should be ISO-8601 character format, and stored as dates in databases
• Some NA dates are currently character "NA", at least the way they are saved as CSVs and loaded.
• date_event_resolved has some rows with NA that should have resolution dates because their report type is "...(Final notification)" and/or the "future reporting" field indicates the event is resolved.
• We'll should have an imputed version of date_event_resolved for reports that are missing them. In some case, regular reporting stops but the disease becomes endemic. In this case we should look to the annual reports to see if the disease continued and for how long. If it's not in the annual reports, it may be that the follow-up report just wasn't submitted and we can impute that it ended a month after the last follow-up, perhaps. "Ongoing" will be events whose last, non-final follow-up was in the past month, though that doesn't have to live in the database, but be part of a run-time query.

[emmamendelsohn]

This is complete with the exception of handling endemic diseases. Since this involves other tables in the database (annual reports), I'll migrate this issue to repel-infrastructure.