fix: fix autocomplete in voiceover+safari

[nolanlawson]

fixes #1629

Follow-up to #1630. I finally got Safari+VoiceOver working (mostly).

The trick was to not use an aria-label on the role="option" element, and to instead just include a <span> positioned offscreen (screen-reader-only class). Apparently Safari+VoiceOver doesn't see the aria-label on the option.

[nolanlawson]

I say "mostly working" because Safari+VoiceOver is still bizarre about not announcing the first selected result when it appears, and also sometimes seems to get into a state where it just doesn't announce anything until you refresh the page. It seems to have something to do with the screenreader focus and the actual focus getting out of sync with each other.

I also confirmed that this PR doesn't break Chrome+VoiceOver, Chrome+NVDA, or Firefox+NVDA.

[nolanlawson]

So for those keeping score at home:

• Safari+VoiceOver was busted because I was using aria-label on the role="option" element instead of just the text inside the element.
• Chrome+NVDA was busted because instead of changing the aria-activedescendant to point to the ID of the selected element, I was changing the ID of the selected element and keeping aria-activedescendant the same.

Firefox+NVDA worked fine with both of these.