FEATURE: Use Postgres unaccent to ignore accents (#16100)

The search_ignore_accents site setting can be used to make the search
indexer remove the accents before indexing the content. The unaccent
function from PostgreSQL is better than Ruby's unicode_normalize(:nfkd).

db/migrate/20220304162250_enable_unaccent_extension.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class EnableUnaccentExtension < ActiveRecord::Migration[6.1]
+  def change
+    enable_extension 'unaccent'
+  end
+end