Improve the private site login form.  

#2378 

@rbran100 can you review this?

Private site login form should now load in the vsite itself.  purl_goto has been removed from vsite.inc and spaces is now handling the access check.  

The private site login should now respect where a user was on a private site when logging out or if they clicked a link to a private site from somewhere else (i.e. I send you a link to a publication/foo on my site which is private) rather than just redirect them to the homepage of the private site.