Use earliest symptom onset to determine how many days of TCN to report symptoms to

[danamlewis]

Idea is to use the earliest symptom onset + 2 days prior to send exposure alerts to the individuals seen in that time period.

Current implementation in 0.4 is for symptom reporting for day-of symptoms and after (?), but for 0.5 we'd like to implement to include the prior 2 days when someone was potentially infectious.