tags: add trigram index on tag names.

db/migrate/20171127195124_add_trigram_index_to_tags.rb

@@ -0,0 +1,9 @@
+class AddTrigramIndexToTags < ActiveRecord::Migration
+  def up
+    execute "create index index_tags_on_name_trgm on tags using gin (name gin_trgm_ops)"
+  end
+
+  def down
+    execute "drop index index_tags_on_name_trgm"
+  end
+end