pundit: convert user feedbacks to pundit.

Allow users to delete feedbacks they've given to other users, not just
mods.

app/policies/user_feedback_policy.rb

@@ -0,0 +1,25 @@
+class UserFeedbackPolicy < ApplicationPolicy
+  def create?
+    unbanned? && user.is_gold? && record.user_id != user.id
+  end
+
+  def update?
+    create? && (user.is_moderator? || (record.creator_id == user.id && !record.is_deleted?))
+  end
+
+  def show?
+    !record.is_deleted? || can_view_deleted?
+  end
+
+  def can_view_deleted?
+    user.is_moderator?
+  end
+
+  def permitted_attributes_for_create
+    [:body, :category, :user_id, :user_name]
+  end
+
+  def permitted_attributes_for_update
+    [:body, :category, :is_deleted]
+  end
+end