[web] Fix several raw html flags in twig (#4325)

* [Update, Config] Add subject for watchdog emails

* [Web] Fixed sender_acl_disabled var in twig

* [Web] Fixed add_relayhost_hint var in twig

* [Web] Fixed merged_vars_hint var in twig

Co-authored-by: Valentin <vbrandner.itsb-b2018@fh-salzburg.ac.at>
Co-authored-by: Valentin <git@voidl.de>