disapprovals: fix searching by user.

Fix searching post disapprovals by user to use the new `visible_for_search`
mechanism. This fixes it so you can do subsearches on the user field, like this:

* https://danbooru.donmai.us/post_disapprovals?search[user][level]=32 (find disapprovals by Builder-level approvers)

app/policies/post_disapproval_policy.rb

@@ -9,6 +9,10 @@ class PostDisapprovalPolicy < ApplicationPolicy
     user.is_moderator? || record.user_id == user.id
   end
 
+  def can_search_creator?
+    user.is_moderator?
+  end
+
   def permitted_attributes_for_create
     [:post_id, :reason, :message]
   end
@@ -30,4 +34,13 @@ class PostDisapprovalPolicy < ApplicationPolicy
     attributes -= [:user_id] unless can_view_creator?
     attributes
   end
+
+  def visible_for_search(disapprovals, attribute)
+    case attribute
+    in :user | :user_id if !can_search_creator?
+      disapprovals.where(user: user)
+    else
+      disapprovals
+    end
+  end
 end