[A11Y] Accessibility improvements for the Search component

[davwheat]

Progresses #3365

Changes proposed in this pull request:

• Simplify JSX using && instead of ternaries
• Add aria-label to role=search element (see MDN)
• Add accessible label to clear search button
• Mark results as live-updating with aria-live
• Improve focus ring on search clear button (would look bad after [A11Y] Adds missing focus rings back to control elements #3016 because of element padding)

Reviewers should focus on:

Are there any other a11y changes that need to be done to this component that I've forgotten?

Confirmed

• Frontend changes: tested on a local Flarum installation.