feat(DTFS2-7621): add amendments url helper

[BethThomas141]

Introduction ✏️

We've written out the URL for different amendments pages a lot over the amendments controllers. This new helper reduces some of the repetition.

Resolution ✔️

• Add amendments URL helper to avoid having to keep hardcoding amendments URL strings.
• Add correct cancel URL to What needs to change? page inline with the other pages in the flow
• Convert the component test for What needs to change? page to Typescript, consistent with the other pages in the flow

Miscellaneous ➕

List any additional fixes or improvements.