Only call BP_Group_Extension::create_screen* methods on proper creation step

By moving the bp_is_group_admin_screen() check into BP_Group_Extension, we
leave one fewer thing for plugins to miss in their implementations.

Load order issues mean that the calls to create_screen() and
create_screen_save() must be moved into standalone callbacks, and hooked
late in the load order so that we have access to information about the current
group creation step.

See #4955

git-svn-id: https://buddypress.svn.wordpress.org/trunk@6988 cdf35c40-ae34-48e0-9cc9-0c9da1808c22