Clean up Govspeak validation

For some time, the `HtmlValidator` has validated the string you pass it as if it were Govspeak. This code gave the impression that it instead treated it as HTML. The intention here was originally to avoid false positives for content which wouldn't be passed through Govspeak, but happened to have content that has meaning in Govspeak. This is apparently not an issue.

I'll look at improving the name of the `Govspeak::HtmlValidator` class separately.