fix pageviews_by_document to properly handle article locale

[escattone]

mozilla/sumo#1896

The root of the problem was my earlier change to use the actual locale of the KB article displayed to the user (AKA the article_locale) instead of the locale in the URL of the KB article requested (see

kitsune/kitsune/sumo/googleanalytics.py

Line 312 in 4cc50d8

article_locale = article_locale if article_locale in VALID_LOCALES else url_locale

). This caused the pageviews_by_document function to return multiple rows for the same (article_locale, slug) pair, which the WikiDocumentVisits.reload_period_from_analytics method wasn't expecting. It only used the last row encountered, so the visits count for many of the WikiDocumentVisits entries was way off.

This PR resolves that by fixing it within the pageviews_by_document function itself, which now returns a dictionary rather than a generator.