Make sure to get the subdomain base on Multisite configs

Move the function to get the subdomain base out of the blogs component to be sure it will be retrieved (when registering a new blog) even if the blogs component is not active.

fixes #5530

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8683 cdf35c40-ae34-48e0-9cc9-0c9da1808c22