search: optimize related tag calc in multi-tag searches (#4120).

In multi-tag searches, we calculated the tags in the sidebar by sampling
300 random posts from within the search and finding the most frequently
used tags. This meant we were effectively doing two searches on every
page load, one for the actual search and one for the sidebar. This is
not so bad for fast searches, but very bad for slow searches.

Instead, now we calculate the related tags from the current page of
results. This is much faster, at the cost of slightly lower accuracy and
the tag list changing slightly as you browse between pages.

We could use caching here, which would help when browsing between pages,
but we would still have to calculate the tags on the first page load,
which can be very slow in the worst case.

app/presenters/post_set_presenters/post.rb

@@ -29,7 +29,7 @@ module PostSetPresenters
       elsif Tag.has_metatag?(post_set.tag_array, *Tag::SUBQUERY_METATAGS)
         calculate_related_tags_from_post_set
       else
-        related_tags_for_group
+        calculate_related_tags_from_post_set
       end
     end
 
@@ -46,7 +46,10 @@ module PostSetPresenters
     end
 
     def related_tags_for_group
-      RelatedTagCalculator.calculate_from_sample_to_array(post_set.tag_string).map(&:first)
+      normalized_tags = Tag.normalize_query(post_set.tag_string, normalize_aliases: false)
+      Cache.get("PostSetPresenters::Post#related_tags_for_group(#{normalized_tags})", 5.minutes) do
+        RelatedTagCalculator.calculate_from_sample_to_array(normalized_tags).map(&:first)
+      end
     end
 
     def related_tags_for_single(tag_string)