login-dialog: Increase top/bottom padding

The login dialog has a maximum height set to prevent the user list
from growing indefinitively. However this approach fails if the
monitor height is smaller as said maximum, so add some additional
top/bottom padding to make sure there's some whitespace above/below
the user list in that case as well.

https://bugzilla.gnome.org/show_bug.cgi?id=685851

data/theme/gnome-shell.css

@@ -2215,6 +2215,9 @@ StScrollBar StButton#vhandle:active {
     border: none;
     background-color: transparent;
 
+    padding-bottom: 80px;
+    padding-top: 80px;
+
     border-radius: 16px;
     min-height: 150px;
     max-height: 700px;