Fixes issues of #17255. Fix tab sequence

[nico-amsterdam]

awesomplete.js:

• Use tabindex -1 instead of 0 to make elements focusable. This does not disturb the tab sequence. Tested with the Talkback screen reader.
• Make aria-activedescendant empty when none is selected. This is when
  • none of the items has aria-selected=true
  • or the popup is closed

awesomplete.theme.css

• After the transition of 0.3 seconds the visibility should be hidden. Prevents navigation to anything inside the now hidden popup.

awesomplete.css is updated by gulp because of the change in awesomplete.theme.css

[dpb-bah]

Confirmed this works. We've been using awesomplete for a while but it flags issues with AXE scans for Section 508 compliance. This PR resolves it.

[LeaVerou]

Thanks for checking it out @dpb-bah and for contributing @nico-amsterdam!