Add unconfirmed_email to spree_users

Up until now, we have not been adding the unconfirmed_email column
to spree_users table. However, this column is required by default
when using Devise's :confirmable module, which also activates Devise's
:reconfirmable behavior by default, which in turn requires
unconfirmed_email.

db/migrate/20200417153503_add_unconfirmed_email_to_spree_users.rb

@@ -0,0 +1,7 @@
+class AddUnconfirmedEmailToSpreeUsers < ActiveRecord::Migration[5.2]
+  def change
+    unless column_exists?(:spree_users, :unconfirmed_email)
+      add_column :spree_users, :unconfirmed_email, :string
+    end
+  end
+end