Bump govuk_content_models to 5.0.0

This is to pick up the Attachable attribute and the `caption_file`
field that's been added to VideoEdition.
Add captions to artefacts if they have them

VideoEditions may provide caption files for videos in the form of XML
files. These XML files are stored elsewhere, but provide a
web-friendly URL to download the raw XML file.
Merge pull request #102 from alphagov/attach-caption-files

Attach caption files