Simplify call to Govspeak::HtmlValidator

Use the #invalid? method instead of an easy-to-miss bang.
Use Govspeak's image source validation

This means it won't be presented as a separate message, but on the other hand
we will no longer convert every string field in a request into HTML twice.
Merge pull request #55 from alphagov/more_efficient_image_validation

More efficient image validation