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