Styles and stories for the event card

[helenb]

This adds the new styles and stories needed for the event card.

• It adds a new card in the story book list called event
• It adds a new variant for laying out the card at desktop so the image fills one third - 'card--horizontal-thirds'
• Adds overrides to existing card styles for the event card
• Adds 'highlight', 'event type' and 'event descriptor' fields
• Adds dark mode variations for the event card

[helenb]

@ahosgood here are my code changes for the event card - it seems to be failing a lot of tests which I suspect are fixtures files needing updating. I will have a look at this later as I want to focus now on getting this into ds-wagtail. However feel free to review these changes and assess if you think they would be useful.

[helenb]

@ahosgood would you like me to update this now that a few things have been refactored on the ds-wagtail side? If not it might be best just to close this now.

[ahosgood]

@helenb - I think it might be best to just keep it all in ds-wagtail for now. If it helps you to develop using Storybook then carry on but I don't think we should merge it in to here until everything is finalised.