ensured autocomplete fields do not accidentally trigger form submission

this seems crude and something that jQuery UI should include, but
apparently not - at least not in the current version
upgraded jQuery UI

general maintenance; includes various bugfixes (though nothing in
particular)

as before (cf. 400aa9a641bffb16399548900d60c509f9f049e5), the upgrade
docs have not been updated because the procedure likely isn't stable -
also, original CSS files were modified to add `.scss` suffix and replace
`url(...)` with `asset-url("...")` (except for `data:` URIs)
fixed handling of empty result sets in autocomplete

latest jQuery UI doesn't like `label` being `undefined`

abusing the results list to display errors doesn't seem like a great
idea in the first place...