EXT-2232: Arghhh! Code Duplication == Bugs.

Apparently, pre-login notifications use LLAlertDialog, but post-login
notifications use LLToastAlertPanel. The latter is basically a copy
and paste of the former, with some local modifications. However, the
person who copy/pasted the code did not initialize the URLLoader
callback, so all notifications that tried to open a web page on a
button click after login were failing.

I've added comments that this code should be refactored, preferably as
a subclass of LLAlertDialog. In the meantime, I've tried to clean it
up a bit by not duplicating the nested URLLoader class (which would've
required further duplication to reimplement the exact same loader class
that LLAlertDialog uses). This let me then initialize the URLLoader
callback for LLToastAlertPanel, to fix this specific bug.

Again... arghhh!!