BP 1.7 compatibility

Because BP1.7 now uses overloading to get and set properties on the BuddyPress object, you can't modify array keys of a property directly ( `$bp->welcome_pack['key']` loads `__get` on `welcome_pack`, which isn't passed by reference..).

As a quick fix I turned `$bp->welcome_pack` into an on-the-fly object, so we can access it directly. Not a long term solution, but enough to get Welcome Pack emails working again.