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?
nope any  but used to declare instance var inside the implementation section {} prefixed with _