FEATURE: Display the reason for many reviewable items

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

db/migrate/20190411144545_add_reason_to_reviewable_scores.rb

@@ -0,0 +1,5 @@
+class AddReasonToReviewableScores < ActiveRecord::Migration[5.2]
+  def change
+    add_column :reviewable_scores, :reason, :string
+  end
+end