post disapprovals: refactor disapproval reasons.

* Factor out reasons into a constant
* Change column default and eliminate unused `legacy` reason.

db/migrate/20200403210353_change_reason_default_on_post_disapprovals.rb

@@ -0,0 +1,6 @@
+class ChangeReasonDefaultOnPostDisapprovals < ActiveRecord::Migration[6.0]
+  def change
+    change_column_null :post_disapprovals, :reason, false
+    change_column_default :post_disapprovals, :reason, from: "legacy", to: nil
+  end
+end