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.
currently, this is the list of allowed HTML tags:

```ruby
["b", "em", "i", "strong", "u", "a", "abbr", "blockquote", "br", "cite", "code", "dd", "dfn", "dl", "dt", "kbd", "li", "mark", "ol", "p", "pre", "q", "s", "samp", "small", "strike", "sub", "sup", "time", "ul", "var", "address", "article", "aside", "bdi", "bdo", "body", "caption", "col", "colgroup", "data", "del", "div", "figcaption", "figure", "footer", "h1", "h2", "h3", "h4", "h5", "h6", "head", "header", "hgroup", "hr", "html", "img", "ins", "main", "nav", "rp", "rt", "ruby", "section", "span", "style", "summary", "sup", "table", "tbody", "td", "tfoot", "th", "thead", "title", "tr", "wbr"]
```

I don't know whether this needs to be expanded or reduced...