Don't treat section descriptions as raw HTML.

We don't need this any more, since none of the section descriptions
coming from the content API contain escaped HTML entities any more.
Don't render raw descriptions on section pages.

Also bump gds-api-adapters to 5.1.0 to get new functionality in the
content API test helpers to test descriptions.
Merge pull request #245 from alphagov/fix-encoding-in-tags

Fix raw display of section descriptions