Fix duplicate groupblog activity post entries.

Needed to look up existing activity entry with proper activity type.

Also:

* Add a different activity action if we are editing an existing blog post.
* Use the group's privacy setting for the activity 'hide_sitewide' flag

Fixes #25.
Add activity comment check for groupblog posts.
Use post permalink for groupblog post activity permalink.
Readme updates
Bumps to 1.8.4
Merge branch '1.8.x'