Don't show notifications from inactive components in admin bar

Inactive components don't have formatting callbacks registered, which means
that those notifications will not be properly formatted. In addition, the
target links of the notifications will 404, with the result that the
notifications will not be marked as read when clicked.

Fixes #5290

Props imath

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7677 cdf35c40-ae34-48e0-9cc9-0c9da1808c22