Update Aside in skeleton template to be more accessible

[lb-]

WHY are these changes introduced?

The current Aside component will read 'times' (or even nothing) instead of 'close' when screen readers navigate to the close button.

While there are other accessibility issues with this template component, this is a quick win and it would be good for developers to get a head start with accessibility where possible.

WHAT is this pull request doing?

Adds aria-label to advise the button element of a custom label that's different to the element's content.

HOW to test your changes?

Set up the skeleton template to output locally, observe the close button in any Aside (such as the search dialog).

Before

Reminder: Some screen readers will not read anything for the 'times' symbol, but if they do they can read out things like 'multiply'.

After

Checklist

• [X] I've read the Contributing Guidelines
• [X] I've considered possible cross-platform impacts (Mac, Linux, Windows)
• [X] I've added a changeset if this PR contains user-facing or noteworthy changes
• [ ] I've added tests to cover my changes N/A
• [] I've added or updated the documentation N/A

References

• https://www.deque.com/blog/dont-screen-readers-read-whats-screen-part-1-punctuation-typographic-symbols/ (explains how different characters are read)

[lb-]

Thanks for reviewing & merging (inc. my other PRs).