Prevent 422 status code with exception for invalid CSRF tokens

When the user tries to submit a form with invalidated session, this fix
will prevent it from raising an error
Log to Logstash when authenticity toke is invalid

Configured it for Test environment so all the tests pass as expected.
Merge pull request #427 from ministryofjustice/76785866-csrf

Prevent 422 status code with exception for invalid CSRF tokens