modreports: fix reporting inaccessible dmails and forum posts.

* Fix it being possible to report dmails belonging to other users.
* Fix it being possible to report forum posts in mod-only threads.

app/models/dmail.rb

@@ -189,7 +189,7 @@ class Dmail < ApplicationRecord
   end
 
   def reportable_by?(user)
-    is_recipient? && !is_automated? && !from.is_moderator?
+    owner == user && is_recipient? && !is_automated? && !from.is_moderator?
   end
 
   def dtext_shortlink(key: false, **options)

app/models/forum_post.rb

@@ -95,7 +95,7 @@ class ForumPost < ApplicationRecord
   end
 
   def reportable_by?(user)
-    creator_id != user.id && !creator.is_moderator?
+    visible?(user) && creator_id != user.id && !creator.is_moderator?
   end
 
   def votable?