Improve buttons on add-user-to-project typeahead

Now if there is an @ sign in the username, it's taken as an email
address that needs to receive an invite. Otherwise if there is
something typed into the search field, the button will say "Add
Existing" unless there are precisely zero results. Only if the
serach field is blank or there are zero results for the term in
the search field will the button say "Create New."