An autocomplete field should only show if the `autocomplete` flag
is set in the function invocation's options hash, or when in the
admin more than 100 users are returned for the query.

Introduce wp.select2, which stores query arguments per dropdown field
instance, which is passed to the user query AJAX call.

Fix some documentation, use boolean defaults rather than `0` or `1`.

First run at l10n.

Allow numerous instances of an autocomplete dropdown.