* Use the tag_list method, rather than the cached_tag_list attribute
This is a follow on to #15456 and #15637
Saving the article in #15637 was a work-around to force the article to
have a valid cached tag list. However, we can rely on the tag_list
method to find the associated tags.
Update the worker to use tag_list (rather than the decorator's
cached_tag_list.array) and flatten the tag list rather than flat
mapping String#split over the non-nil cached_tag_lists.
* Remove unused join on tags
The query logging showed each article's tags were loaded twice in a
row (select by id, immediately followed by select by id). Removing the
join removed this behavior. I suspect mapping `#tag_list` over
articles is ignoring the loaded tags.
Performance (in my local environment) improved about 30% when this join was removed.
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