[skeleton] Close aside when Esc is pressed

[frontsideair]

WHY are these changes introduced?

The aside doesn't follow ARIA APG about dismissing with Esc key

WHAT is this pull request doing?

Adds an event handler to dismiss the aside with the Esc key

HOW to test your changes?

1. Click the search link on the header navigation
2. Press Esc key to dismiss

Post-merge steps

Checklist

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

[scottdixon]

Thanks again @frontsideair. We're in the middle of a release but can merge this shortly after!

[scottdixon]

CI failing on CSP tests which have been fixed. Confirmed tests pass with a rebase. Merging!