Create "Export scores" tab

[jacobbien]

(This is inspired by the very nice "Export Data" page on covidcast: https://delphi.cmu.edu/covidcast/export/)

• User selects some forecasters, aheads, scores, and a time window.
• Provides user with a "Download CSV" or R code snippet that can be copied and pasted.
• What is returned is a data frame with columns geo_value, forecast_date, target_end_date, ahead, and then whatever scores have already been calculated (such as weighted_interval_score, coverageXX, absolute_error... i.e., whatever has already been computed on the backend, pre-aggregation).

Motivation: This will allow power users to make all the aggregates they want without having to go through the slow process of downloading all the individual forecasts and computing scores themselves.

[kateharwood]

Done, in staging currently.