upgraded jQuery UI

apart from general maintenance, this is required for an improved
autocomplete experierence WRT loading indicators (cf. upcoming commits)

note that the upgrade docs are not entirely accurate anymore, but I
opted not to bother adjusting them because those details will most
likely change again with the next upgrade

also note that we had to modify the original CSS file, adding the .scss
suffix and replacing `url(...)` with `asset-url("...")` (except for
`data:` URIs)

/cc @mrreynolds
ensured autocomplete loading indicator is hidden for negative responses