Switch alert show/hide to a prop

Changes proposed in this pull request:

• Switch Alert show-hiding to a prop (this ensures that the aria-live region in the Alert component is present on page load)

Related issues

Closes #531 (closed) Overrides #573

Submitter checklist

• Added logging is not capturing sensitive data and is set to an appropriate level (DEBUG vs INFO etc)
• Updated relevant documentation (README, ADRs, explainers, diagrams)

Security considerations

None, UI refactor only