1. Peter's APM changes broke subsite mounting because the old post
instantiate routine wasn't really called until after the site node for
the subsite was mounted. No problem - I changed it to be an after-mount
callback instead (which was the idea all along)
2. Earlier changes made by me broke the adding of users by an admin, because
the main subsite no longer has a group defined by default separate from
registered users. The registered users group *is* the main subsite
membership group. Had to add a special check for this case.
3. Removed the naughty composition rel added by application_group::new. This
caused subsites to have undesirable side-effects (add admin to subsite and
the user magically became an admin of the supersite, if you defined an
admin_rel and adminstrator relational segment in each)
