Refactor manual JSON schema validation

This introduces a reusable validator and moves JSON schema validation to the model.
Refactor section JSON schema validation
Merge pull request #27 from alphagov/refactor-json-schema-validation-to-activemode-validation

Refactor json schema validation to use ActiveModel validation
Reject any manual or section with HTML text fields

For each submission, the attributes are crawled recursively and
each found text-field is piped through the 'sanitize' gem, so that
the HTML can be detected.