Blogs: Formatting improvements for activity comments connected to blog comments.

This commit:
- Readjusts the permalink for singular activity comments connected to blog comments to use the comment permalink instead of the activity thread permalink
- Readjusts the activity action for singular activity comments connected to blog comments to use the familiar blog comment action instead of the generic "posted an activity comment" action

See #5560 (trunk)

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8345 cdf35c40-ae34-48e0-9cc9-0c9da1808c22