Signups: Introduce functions for upgrading the global `wp_signups` table if an old version exists. This happens primarily on singlesite installations where third party plugins may have previously created the sign-ups table without keeping it updated.

* Bumps the DB version number to 8300.
* Moves functions for table creation and upgrade into the admin schema file.
* Moves sign-ups table creation out of activation process, and into the database upgrade process.
* No not rely on `dbDelta()` for these specific database upgrades (inspired by [WP25179])

See #5563, #WP27855. (trunk)

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8302 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Check get_current_screen() is available before using it

In non multisite configs, to avoid displaying the signups (user_status set to 2) into the users administration screen, a filter to the user query is applyed after checking the current screen is the users one. It appears to be problematic on some configs. So we need to check the function get_current_screen() exists just before using it.

Fixes #5551

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8304 cdf35c40-ae34-48e0-9cc9-0c9da1808c22