User autocomplete: Add the class autocompletes-user for easier selecting later

This is nice because it means you can select/iterate all user
autocompleters using basic jQuery / CSS.

For the bespoke implementations, I add the class from JS so that if JS
doesn't run, the elements aren't tagged incorrectly.  That really only
matters if you're selecting elements with CSS (since your custom JS
won't be running either).
User autocomplete: Override the item render to conditionally use HTML from the server

Combined with the recent ModifySuggestion callback for
/Helpers/Autocomplete/Users, this lets customizers fully affect
rendering of the autocomplete suggestions.
Merge branch '4.0/user-autocomplete-extensibility' into 4.0-trunk