Smooth CSS transition for FxA popup (bug 1074348)

https://bugzilla.mozilla.org/show_bug.cgi?id=1074348

No gifs for this one because they drop the unstyled content flash.

Before: https://www.dropbox.com/s/t1c9fbf5nzw7s9j/reload-before.mov?dl=0
After: https://www.dropbox.com/s/2hvmvqrmbau415c/reload-after.mov?dl=0

If I did this in JavaScript I'd need to loop through all the children in `body` and remove the ones that aren't `main` taking care not to remove any scripts or stylesheets (or something else useful) that may be in there.

I really just want to hide the visual elements so I think this makes the most sense.