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

test/unit/concerns/searchable.rb

@@ -127,6 +127,9 @@ class SearchableTest < ActiveSupport::TestCase
         assert_search_equals(@wp, other_names_include_any_lower_array: ["A1", "BLAH"])
         assert_search_equals(@wp, other_names_include_all_lower_array: ["A1", "B2"])
 
+        assert_search_equals(@wp, any_other_name_matches_regex: "^a")
+        assert_search_equals(@wp, any_other_name_matches_regex: "[a-z][0-9]")
+
         assert_search_equals(@wp, other_name_count: 2)
       end
     end