Add e-mail-based sign in challenge for users with disabled 2FA (#14013)

db/migrate/20200608113046_add_sign_in_token_to_users.rb

@@ -0,0 +1,6 @@
+class AddSignInTokenToUsers < ActiveRecord::Migration[5.2]
+  def change
+    add_column :users, :sign_in_token, :string
+    add_column :users, :sign_in_token_sent_at, :datetime
+  end
+end