function Meteor.userLoaded(), which is true if you are logged in and the user
doc is loaded, and a currentUserLoaded Handlebars helper to match.
If logged in and the user doc is not yet loaded, Meteor.user() now returns an
object which only contains _id.
The current user subscription is now named meteor.currentUser rather than being
an unnamed sub. (loginServiceConfiguration is renamed
meteor.loginServiceConfiguration to match.) This subscription is sub'd from when
you log in and unsub'd from when you log out (or if you log in with different
credentials).
I was very careful to make sure that in the case of "sub #1, unsub #1, sub #2,
sub #1 is ready" we do not declare the user to be ready. I could have instead
modified livedata_connection to not call ready callbacks for unsub'd
subscriptions (add a "delete self.sub_ready_callbacks[obj._id]" to the self.subs
removed function) but this seemed less invasive.
The password and email tests use this to take a more rigorous approach to
waiting for the data to load, and they change the localStorage keys so that
multiple tabs running tests don't interact via localStorage.
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