Move `DO_NOT_UPGRADE_GLOBAL_TABLES` check from `bp_core_maybe_install_signups()` to `bp_core_upgrade_signups()`. Fixes bug where `wp_signups` table would not get created on single-site installations when `DO_NOT_UPGRADE_GLOBAL_TABLES` is set. Fixes #5745.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8565 cdf35c40-ae34-48e0-9cc9-0c9da1808c22