Create User dialog no longer automatically suggests username based on first and last names

Note that this change has been discussed during the year at https://forum.xwiki.org/t/proposal-for-a-better-registration-process/13805

Autocompletion is only triggered when we get back to the username field, which makes it way more difficult to use.
If we fix it to trigger as soon as we have a value in both first and last name, I personally think that the regression in usability for this use case would be low enough to justify the changes that improve other use cases (e.g. creating users with just a userName and not setting their first/last names).

Just throwing ideas, but maybe we'd want to setup the inverse autofill: first and last name from the userName. Probably difficult to make something reliable though.