Fixes a regression in r8821 in `rbp_core_load_template()` which was preventing the correct parent page being highlighted in WP nav menus.

This change reinstates the page look up and post globals as before r8821. Overall, these changes are still a net benefit as an uncached SQL query has been removed. See #5241 for background.

Fixes #5810

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8932 cdf35c40-ae34-48e0-9cc9-0c9da1808c22