* Extract repeated calls to cache_bust.call to iteration over list
This mirrors what the EdgeCache::BustUser service looks like.
It might make sense to have cached_article_paths be a method accepting
a block and yielding paths, and including the collections cache paths
as well.
* Move article path information into a separate method
BustArticle.call now only knows to bust cache for each path associated
with the article, the information about which paths to clear is moved
to a separate `paths_for` method.
I may have gone overboard on this.
* Remove extra blank line
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