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/structure.sql

@@ -352,6 +352,15 @@ CREATE FUNCTION public.favorites_insert_trigger() RETURNS trigger
       $$;
 
 
+--
+-- Name: reverse_textregexeq(text, text); Type: FUNCTION; Schema: public; Owner: -
+--
+
+CREATE FUNCTION public.reverse_textregexeq(text, text) RETURNS boolean
+    LANGUAGE sql IMMUTABLE PARALLEL SAFE
+    AS $_$ SELECT textregexeq($2, $1); $_$;
+
+
 --
 -- Name: testprs_end(internal); Type: FUNCTION; Schema: public; Owner: -
 --
@@ -388,6 +397,17 @@ CREATE FUNCTION public.testprs_start(internal, integer) RETURNS internal
     AS '$libdir/test_parser', 'testprs_start';
 
 
+--
+-- Name: ~<<; Type: OPERATOR; Schema: public; Owner: -
+--
+
+CREATE OPERATOR public.~<< (
+    FUNCTION = public.reverse_textregexeq,
+    LEFTARG = text,
+    RIGHTARG = text
+);
+
+
 --
 -- Name: testparser; Type: TEXT SEARCH PARSER; Schema: public; Owner: -
 --
@@ -7849,6 +7869,7 @@ INSERT INTO "schema_migrations" (version) VALUES
 ('20210106212805'),
 ('20210108030722'),
 ('20210108030723'),
-('20210108030724');
+('20210108030724'),
+('20210110015410');