Add influenza hospitalization forecast evaluations to the dashboard and pipeline

[nmdefries]

Flu forecasts can be fetched from cdcepi/Flusight-forecast-data.. Additional info at cdcepi/FluSight-forecasts and https://www.cdc.gov/flu/weekly/flusight/flu-forecasts.htm.

Flu hosp truth is the hhs data, which we have in the covidcast endpoint. Some extra processing is required due to it using week-based/7-day-sum targets.