Performer custom fields (#5487)

* Backend changes
* Show custom field values
* Add custom fields table input
* Add custom field filtering
* Add unit tests
* Include custom fields in import/export
* Anonymise performer custom fields
* Move json.Number handler functions to api
* Handle json.Number conversion in api

pkg/sqlite/migrations/71_custom_fields.up.sql

@@ -0,0 +1,9 @@
+CREATE TABLE `performer_custom_fields` (
+  `performer_id` integer NOT NULL,
+  `field` varchar(64) NOT NULL,
+  `value` BLOB NOT NULL,
+  PRIMARY KEY (`performer_id`, `field`),
+  foreign key(`performer_id`) references `performers`(`id`) on delete CASCADE
+);
+
+CREATE INDEX `index_performer_custom_fields_field_value` ON `performer_custom_fields` (`field`, `value`);