Use `validate` in apply-tier-4-visa
Use `validate` in apply-tier-4-visa

I've made use of a `next_node_calculation` block (which gets run before the
`validation` block and has access to the `response`) to avoid introducing
duplication in the `validate` and `calculate` blocks.

askl56/smart-answers,stwalsh/smart-answers,alphagov/smart-answers,askl56/smart-answers,aledelcueto/smart-answers,alphagov/smart-answers,stwalsh/smart-answers,tadast/smart-answers,askl56/smart-answers,tadast/smart-answers,alphagov/smart-answers,tadast/smart-answers,alphagov/smart-answers,tadast/smart-answers,stwalsh/smart-answers,aledelcueto/smart-answers,stwalsh/smart-answers,aledelcueto/smart-answers,aledelcueto/smart-answers,askl56/smart-answers