Reject payloads with disallowed HTML

This builds upon #35, which will need to be merged first.
The validator is called from the model with this call:

```ruby
validates :to_h, no_dangerous_html_in_text_fields: true
```

Capitalising HTML in the validator class name trips up rails auto-loading, and would mean having to do some workaround in the model class, which doesn't feel like a worthwhile trade-off...