Add preference for report notification e-mails, skip for duplicates (#8559)

If an unresolved report for the same target account already exists,
no new notification is generated

config/settings.yml

@@ -39,6 +39,7 @@ defaults: &defaults
     mention: false
     follow_request: true
     digest: true
+    report: true
   interactions:
     must_be_follower: false
     must_be_following: false