Add a variable for contact email address incl subject

* Additional variable serves as contact email address that includes the
email subject already in the link.

_config.yml

@@ -44,6 +44,7 @@ gitlab:
   project_url: https://gitlab.hzdr.de/hifis/software.hifis.net
 
 contact_mail: support@hifis.net
+contact_mail_for_gitlab: "support@hifis.net?subject=[GitLab]: Add Request description"
 helpdesk: support.hifis.net
 
 sass: