Change to community logins

I've done some minor hacking on some changes to the community login
system, based on nagging (eh, sorry, feedback) from mainly JD ;-)

Specifically, not using http auth, but instead using forms auth and a
session cookie. This is so we can eventually have a separate login link
for those who don't understand systems that ask you to login only when a
login is necessary.

It will also make it possible to have pages change depending on if
you're logged in or not - but that requires code to run on wwwmaster,
which we're trying to avoid, so I'm not sure much of that will happen...

Anyway. Attached is a patch that does this, you can test the system out
on http://magnus-master.pgadmin.org. Thoughts and comments?

//Magnus