Refactor quiz answers to avoid overriding Source styles

Due to differences in design, quiz answers currently override a number of the Source radio button's default styles. As this relies upon the existing styling and structure of the component it is liable to break when a new version of Source is released.

Some possible options to avoid this:

- Extend the radio button component's theming support to allow more extensive customisation.
- Don't provide a label to the radio button component so it _only_ renders the button. This could then be wrapped with a `label` element which can be styled as needed.
- Don't use the radio button component at all. If the design has deviated enough from the default styles then perhaps it should be a separate component.


Start by locating the quiz answer implementation and its use of the Source radio button component, then compare the answer styling with Source's default styles. Review the three proposed approaches and confirm the result no longer overrides Source styles while preserving the quiz answer design.

Written by the indexing model from the issue text.