Remove the bp_get_signup_allowed() checks in Sign-ups management feature

- Create $wpdb->signups table even if sign-ups are not allowed on single blog config
- Display the UI to manage sign-ups even if signups are not allowed

This way, administrator who temporarly deactivated registrations will still be able to moderate the pending signups.

props johnjamesjacoby, boonebgorges

See #5374

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8196 cdf35c40-ae34-48e0-9cc9-0c9da1808c22