Implements the standard quality heuristic described in https://phabricator.wikimedia.org/T332383 as a feature metric,
- the 'standard_quality' is boolean for the metric features dataset, e.g. wiki_db,page_id,time_bucket,standard_quality
- for the content gap metrics, the 'standard_quality' is the % of articles that above the threshold for a given gap category/timebucket
Also implemented is a forward fill for missing article quality values, though this will be further refined in a future MR.