webui: use asynchronous call for authentication
    
    Change `IPA.login_password` and `IPA.get_credentials` to use async AJAX
    and to return promise instead of blocking the code.
    
    IPA.get_credentials is still partially blocking because of negotiate process.
    We can't do anything about that.
    
    It allows activity indicators to do their job.
    
    https://fedorahosted.org/freeipa/ticket/3903
    
    Reviewed-By: Adam Misnyovszki <amisnyov@redhat.com>