Reintroduce bp_blogs_record_existing_blogs() to bp_core_activation_notice()

It was originally removed in r8592 because it was taxing to resources, but

Fixes #5820. See #5825

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8946 cdf35c40-ae34-48e0-9cc9-0c9da1808c22