replace with progressview upon login button click as suggested by tkriz

as suggested by @tkriz let's add a UIActivityIndicatorView in-place of the login-button when clicked

done for [AGPUSH-899](https://issues.jboss.org/browse/AGPUSH-899)

@sebastienblanc @corinnekrych @matzew mind to give it a spin?
Why not have it as a private (inside .m file) @property? Does it have any advantages?