Blogs: Support post comment synchronization in the activity stream.

Previously, if a reply to a blog post was made in the activity stream,
the reply would only exist in the activity stream and not as a comment
for the blog post.

This commit:
- Adds a blog comment whenever an activity reply is made under a blog post activity entry
- Adds an activity comment to the blog post's activity entry whenever a blog comment is made
- Checks to see if the blog post is open for comments (whether manually closed or due to age) (see r8189) to determine whether the activity item can be replied to
- Handles content edit synchronization to either the post comment or the activity comment
- Handles deletion synchronization to a degree

See #5130

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8190 cdf35c40-ae34-48e0-9cc9-0c9da1808c22