search: add way to search array attributes by regex.

Add a `where_any_in_array_matches_regex` method and expose it to the API:

 * https://danbooru.donmai.us/artists?search[any_other_name_matches_regex]=^blah
 * https://danbooru.donmai.us/wiki_pages?search[any_other_name_matches_regex]=^blah
 * https://danbooru.donmai.us/saved_searches?search[any_label_matches_regex]=^blah

In SQL, this does `WHERE '^blah' ~<< ANY(other_names)`, where `~<<` is a
custom operator based on the `~` regex match operator, but with the
arguments reversed. This allows it to be used with the ANY(array) operator.

See also:

* https://stackoverflow.com/a/22101172
* https://www.postgresql.org/docs/current/sql-createfunction.html
* https://www.postgresql.org/docs/current/sql-createoperator.html
* https://www.postgresql.org/docs/current/functions-comparisons.html

db/migrate/20210110015410_add_reverse_regex_operator.rb

@@ -0,0 +1,11 @@
+class AddReverseRegexOperator < ActiveRecord::Migration[6.1]
+  def up
+    execute "CREATE FUNCTION reverse_textregexeq (text, text) RETURNS boolean LANGUAGE sql IMMUTABLE PARALLEL SAFE AS $$ SELECT textregexeq($2, $1); $$"
+    execute "CREATE OPERATOR ~<< (FUNCTION = reverse_textregexeq, leftarg = text, rightarg = text)"
+  end
+
+  def down
+    execute "DROP OPERATOR ~<< (text, text)"
+    execute "DROP FUNCTION reverse_textregexeq (text, text)"
+  end
+end