Refactor ID card sign-in

- Extract to Devise custom strategy
- Use `SSL_CLIENT_S_DN_CN` env variable instead of `SSL_CLIENT_S_DN` to
get ID card data
- Remove `database_authenticatable` strategy from `RegistrantUser`

Closes #1047

test/lib/devise/strategies/id_card_authenticatable_test.rb

@@ -0,0 +1,13 @@
+require 'test_helper'
+
+class IdCardAuthenticatableTest < ActiveSupport::TestCase
+  def test_valid_when_id_card_data_is_present_in_env
+    strategy = Devise::Strategies::IdCardAuthenticatable.new({ 'SSL_CLIENT_S_DN_CN' => 'some' })
+    assert strategy.valid?
+  end
+
+  def test_not_valid_when_id_card_data_is_absent_in_env
+    strategy = Devise::Strategies::IdCardAuthenticatable.new({})
+    assert_not strategy.valid?
+  end
+end