Markdown => HTML in 'body' and 'description'

`kramdown` is used to convert the markdown. However the `govspeak` gem is included
in the dependency, because HTML sanitisation will use this gem.
Document markdown => html conversion in README
Merge pull request #35 from alphagov/allow-bodies-and-descriptions-to-contain-markdown

Markdown => HTML in 'body' and 'description'
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
Bump gds-api-adapters to 14.0.0

From now on, content will be published to the Publishing API, and
read from the Content Store. All the naming and adapters have been
updated to reflect this split.