Ensure GitLab abuse reports are taken into account

Context: "Handling GitLab abuse reports" email thread on the sysadmins mailing list.

• corresponding doc: https://docs.gitlab.com/ee/user/admin_area/abuse_reports.html
• problem: no "Abuse reports notification email" is set on https://gitlab.tails.boum.org/admin/application_settings/reporting ⇒ nobody is notified when abuse is reported

intrigeri's initial proposal:

• We add "Process GitLab abuse reports" to the sysadmins' mission.

• You folks set the "Abuse reports notification email" to something that enables you to do this work.