Only allow safe Govspeak in SitewideSettings

This is the same validation used in Edition models and enforces a whitelist of supported elements.
Merge pull request #1615 from alphagov/sitewide-settings-govspeak-validation

Only allow safe govspeak in SitewideSettings
Handle empty pages by 404

* Render a 404 instead of an empty services and info page
* Update gds_api_adapters gem to version 12.4.1 to use Rummager test
  helpers
Sort services and info data by title

Adds search query param to get Rummager response alphabetically sorted by title
Adds styling tweaks to "see more" links
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