Blogs: When a blog comment is unapproved, mark the corresponding activity item as spam.

This mirrors how BuddyPress handles other similar comment statuses
like 'trash' and 'spam'.

See #5130

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8229 cdf35c40-ae34-48e0-9cc9-0c9da1808c22