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.

db/migrate/20201213052805_add_extension_fuzzy_str_match.rb

@@ -0,0 +1,5 @@
+class AddExtensionFuzzyStrMatch < ActiveRecord::Migration[6.0]
+  def change
+    enable_extension "fuzzystrmatch"
+  end
+end