Fixed #19464 -- Eased customization of ClearableFileInput's link markup

Thanks rubyruy for the report and the patch.
Fixed #19838 -- Admin: Don't leak a 500 HTTP status when trying to delete protected FKs.

Thanks rafadev for the report and Javier Mansilla for the fix.
Fixed #6585 -- Admin relationship widgets: Respect ordering defined by target model's ModelAdmin.

Thanks Gary Wilson for the report and Juan Pedro Fisanotti, Carlos
Matías de la Torre for the fix.