Manual and section text-field sanitisation

https://www.pivotaltracker.com/n/projects/887836/stories/74137504

This PR adds validation that text fields in manuals and sections don't contain HTML.
No, that's a mistake.

I'm in the process of splitting this PR into two; this one and a [separate one for JSON schema validation](https://github.com/alphagov/hmrc-manuals-api/pull/27) which has this comment removed.