extending callout states and refactored

[DJMyles]

Description

The UI kit at the moment includes a base callout class and a warning variant. This branch seeks to include states for success, error and info, updates the warning variant to use the correct $colour-warning variable and adds a small amount of refactoring by pushing some duplicated attributes to the base callout where possible.

Additional information

N/A

Definition of Done

• [ ] Content/documentation reviewed by Julian or someone in the Content Team
• [ ] UX reviewed by Gary or someone the Design team
• [ ] Code reviewed by one of the core developers
• [ ] Acceptance Testing
  • [ ] Cross-browser tested against standard browser matrix
  • [ ] Tested on multiple devices (TBD)
  • [ ] HTML5 validation (CircleCI)
  • [ ] Accessibility testing & WCAG2 compliance (npm test)
• [ ] Stakeholder/PO review
• [ ] CHANGELOG updated