Suggestions: move the blog checks in `bp_activity_maybe_load_mentions_scripts` to a filter in the core component.

This reorganisation keeps the component-specific mentions checks in those specific components, rather than bundling everything into the Activity component as it had been.

See #5934

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9243 cdf35c40-ae34-48e0-9cc9-0c9da1808c22