Create new api endpoint for longer/seasonal session data

[echappen]

This endpoint would be used for a new bar-chart on analytics.usa.gov: https://github.com/18F/analytics.usa.gov

If we want to change the "Visits Today" bar-chart into something more seasonal, we may need to generate a new report like this, since there isn't an equivalent report in the list.

This query is modeled off USA.gov's Last 30 Days endpoint: https://app-usa-modeast-prod-a01239f-ecas.s3.amazonaws.com/analytics/raw-data/traffic-sources-30-days.json

We can tweak the number of days to last 90, last 365, or whatever length of days we decide.