fix(PPDSC-2164): assistive text

[mstuartf]

PPDSC-2164

What

1. Background:

• aria-describedby for a form input is set to the id of the assistive text element, which is assumed to be present.
• If no assistive text element is present, aria-describedby references a non-existent element.

3. What did you do:

• Updated the assistive text id value in the form input's context from within the AssistiveText component.
• This way the context will only contain an assistive text id if there is an assistive text element present, so aria-describedby will not be set incorrectly.
• Tests for the above.

5. What does the reviewers should expect:

• aria-describedby for a form input should not be set if there is no assistive text element present.

I have done:

• Written unit tests against changes
• Written functional tests against the component and/or NewsKit site
• Updated relevant documentation

I have tested manually:

• The feature's functionality is working as expected on Chrome, Firefox, Safari and Edge
• The screen reader reads and flows through the elements as expected.
• There are no new errors in the browser console coming from this PR.
• When visual test is not added, it renders correctly on different browsers and mobile viewports (Safari, Firefox, small mobile viewport, tablet)
• The Playground feature is working as expected

[newskit-engineering]

You can preview these changes on:
Documentation Site: http://ncu-newskit-docs.s3-website-eu-west-1.amazonaws.com/ppdsc-2164-assistive-text
Storybook: http://ncu-newskit-docs.s3-website-eu-west-1.amazonaws.com/ppdsc-2164-assistive-text/storybook