Deep stringify test data keys so it works with model specs
Validate that manual free-text has no dangerous HTML
Validate that section free-text has no dangerous HTML
Document allowed HTML in README
Bump govspeak gem to 2.0.0

This picks up a [bugfix](https://github.com/alphagov/govspeak/pull/29)
which led to duplicate entries in the `sanitize` configuration.
Merge pull request #37 from alphagov/manual-sanitisation

Reject payloads with disallowed HTML