Manual free-text sanitisation

https://www.pivotaltracker.com/n/projects/887836/stories/74137504

This PR add validation that free-text fields in manuals don't contain dangerous HTML tags.
The `all_freetext_fields_have_no_dangerous_html_tags` and `dangerous?` look like they'll be common to both. Given that we're using ActiveModel, a logical solution might be to use. In which case, the constant would live in that class.