search: drop special case for pixiv urls in source: metatag.

* Drop support for `source:pixiv/artist-name` searches. This was a hack
  that only worked on old pixiv urls that haven't been used for years.
* Replace the old SourcePattern(lower(source)) index with a trigram index.

db/migrate/20190829055758_drop_source_pattern_index_on_posts.rb

@@ -0,0 +1,24 @@
+# see db/migrate/20130328092739_change_source_pattern_index_on_posts.rb
+
+class DropSourcePatternIndexOnPosts < ActiveRecord::Migration[6.0]
+  def up
+    execute "set statement_timeout = 0"
+
+    execute "DROP INDEX index_posts_on_source_pattern"
+    execute "DROP FUNCTION SourcePattern(text)"
+    add_index :posts, "lower(source) gin_trgm_ops", name: "index_posts_on_source_trgm", using: :gin, where: "source != ''"
+  end
+
+  def down
+    execute "set statement_timeout = 0"
+
+    remove_index :posts, name: "index_posts_on_source_trgm"
+    execute "CREATE FUNCTION SourcePattern(src text) RETURNS text AS $$
+               BEGIN
+                 RETURN regexp_replace(src, '^[^/]*(//)?[^/]*\.pixiv\.net/img.*(/[^/]*/[^/]*)$', E'pixiv\\\\2');
+               END;
+             $$ LANGUAGE plpgsql IMMUTABLE RETURNS NULL ON NULL INPUT"
+    execute "CREATE INDEX index_posts_on_source_pattern ON posts USING btree
+             ((SourcePattern(source)) text_pattern_ops)"
+  end
+end