Add 'Grid display' paragraph type

[gosia-mlynarczyk]

Fields:

• Title (plain text, optional)
• Heading level (select list, optional, default h2)
• Grid theme (select list, mandatory, 2 options: Grid - default, and Box)
  • 1 or more Grid display items (Grid display item paragraphs) each with the following fields:
  • 0 or 1 Image (media entity ref. field, image type)
  • Link (URL and link text, both mandatory, with ‘Open URL in a New Window’ checkbox enabled)

TODO: Think of a better name for that paragraph and its options, as it's not obvious what it does

Relates to 'Add Bracknell campaign paragraphs' issue