fix update of GA4 pageview data for questions

[escattone]

mozilla/sumo#1913

Notes

• The Reload Question Traffic Stats weekly cron has been broken for over 5 months. This PR fixes it.
• There are two problems with the currently released code:
  • The kitsune.sumo.googleanalytics.pageviews_by_question() function returns multiple rows for each question ID -- one for each locale that a question was viewed in during the past 365 days -- and that means that more than 700K rows can be returned in prod. In this PR, it returns a dict with one entry for each question ID where the value of that entry is the total pageviews of the question across all locales, which means over 300K entries in prod.
  • The QuestionVisits.reload_from_analytics() method was causing an OOM error because it was building the instance_by_question_id dict for all questions at once. In this PR, that's now done in batches of 30K.
• A very minor variation of the code in this PR has already been successfully tested within the prod environment.

[smithellis]

This is nice! It has a few hard to read spots (your clever use of enumerate, for instance, felt a bit inception-like -- but I couldn't think of a better/easier to read way of doing that).

I didn't run this locally (don't think it would work unless I mocked GA or something), but it looks good to me, and the tests work. Approved!