Pass activity arguments through to bp_activity_thumbnail_content_images()

This allows plugin authors greater flexibility when modifying a post's
thumbnail image as it'll appear in the blog activity stream, by filtering
'bp_activity_thumbnail_content_images'.

Fixes #5060

Props dcowgill

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7222 cdf35c40-ae34-48e0-9cc9-0c9da1808c22