update all signatures with the verification status

db/post_migrate/20170830084744_update_gpg_signatures_verification_status.rb

+class UpdateGpgSignaturesVerificationStatus < ActiveRecord::Migration
+  DOWNTIME = false
+
+  def up
+    GpgSignature.where(verification_status: nil).find_each do |gpg_signature|
+      gpg_signature.gpg_commit.update_signature!(gpg_signature)
+    end
+  end
+
+  def down
+    # we can't revert setting the verification_status, but actually we don't
+    # need to really, as setting this is not a harmful change.
+  end
+end