add script to automatically update source first_story

[rahulbot]

Our sources have a first_story field on them. We should fill that in regularly searching for the oldest (reasonable) date on each source. Smart to re-run every 6 months or so because we sometimes ingest historical data. Easy to implement if we can run a query for each source for dates from 2000 till now and sort by pub_date ASC. Perhaps don't do this for sources that have less than 100 articles because that's an indicator that we don't get regular data from them? A simple data science script so I'm logging it here, but probably best implemented as a Django management command like update-stories-per-week.