Allow to specify a date range on the /pipeline endpoint

[robertodauria]

Currently the /pipeline endpoint only accepts one parameter; year. This means statistics are recalculated for the whole year every time the pipeline is run. This is not necessary since the statistics are calculated on a daily basis, thus a change in a past day doesn't change it's corresponding histogram rows unless the underlying data changed -- and we want to manage such special cases differently anyway. In general, we can just upload the past 1-2 days every day without removing all the previous days' rows and calculating them again.

To do so, the /pipeline endpoint should support the from and to parameters.

The export phase won't need to be changed as a single histogram_daily_stats.json file includes data for a whole year and we can't generate one day at a time.