Add influenza hospitalization forecast evaluations to the dashboard and pipeline

Flu forecasts can be fetched from [cdcepi/Flusight-forecast-data.](https://github.com/cdcepi/Flusight-forecast-data). Additional info at [cdcepi/FluSight-forecasts](https://github.com/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.

Begin with the Flusight-forecast-data and FluSight-forecasts sources linked in the issue, then trace the existing covidcast endpoint handling HHS hospitalization truth. Pay particular attention to the week-based and 7-day-sum target processing. Done means influenza hospitalization forecasts are evaluated through the pipeline and appear in the dashboard.

Written by the indexing model from the issue text.