FEATURE: Display the reason for many reviewable items

Queued Posts and Users will now display a reason why they are in the
review queue.

spec/serializers/reviewable_user_serializer_spec.rb

@@ -6,6 +6,7 @@ describe ReviewableUserSerializer do
   let(:admin) { Fabricate(:admin) }
 
   it "includes the user fields for review" do
+    SiteSetting.must_approve_users = true
     Jobs::CreateUserReviewable.new.execute(user_id: user.id)
     reviewable = Reviewable.find_by(target: user)