fix array_merge error when installed in standalone WP

Replaces https://github.com/cuny-academic-commons/cbox-theme/pull/204 the PR I accidentally added other commits to, which I'll close.

> Single site usage throws array_merge warning in `wp_register_widget_control()`. This PR avoids loading and registering `CBox_BP_Blogs_Recent_Posts_Widget` when not in WordPress multisite.