autocomplete: optimize searching by artist/wiki page other names.

Optimize searches for non-English phrases in autocomplete. These
searches were pretty slow, and could sometimes cause sitewide lag spikes
when users typed long strings of non-English text into the search box
and caused an unintentional DoS.

The trick is to use an `array_to_tsvector(other_names) USING gin` index
on other_names. This supports fast string prefix matching against all
elements of the array. The downside is that it doesn't allow infix or
suffix matches, so we can't support wildcards in general. Wildcards
didn't quite work anyway, since artist and wiki other names can contain
literal '*' characters.

db/migrate/20210110090656_add_array_to_tsvector_index_on_wiki_pages_and_artists.rb

@@ -0,0 +1,6 @@
+class AddArrayToTsvectorIndexOnWikiPagesAndArtists < ActiveRecord::Migration[6.1]
+  def change
+    add_index :wiki_pages, "array_to_tsvector(other_names)", using: :gin
+    add_index :artists, "array_to_tsvector(other_names)", using: :gin
+  end
+end