autocomplete: tune autocorrect algorithm.

Tune autocorrect to produce fewer false positives. Before we used
trigram similarity. Now we use Levenshtein edit distance with a dynamic
typo threshold. Trigram similarity was able to correct large
transpositions (e.g. `miku_hatsune` -> `hatsune_miku`), but it was bad
at correcting small typos. Levenshtein is good at small typos, but can't
correct large transpositions.

app/logical/autocomplete_service.rb

@@ -100,7 +100,7 @@ class AutocompleteService
   end
 
   def tag_autocorrect_matches(string)
-    tags = Tag.nonempty.fuzzy_name_matches(string).order_similarity(string).limit(limit)
+    tags = Tag.nonempty.autocorrect_matches(string).limit(limit)
 
     tags.map do |tag|
       { type: "tag", label: tag.pretty_name, value: tag.name, category: tag.category, post_count: tag.post_count }