Add "new_groupblog_post" activity type to "Posts" dropdown filter option

[r-a-y]

When the "Posts" option is selected in the activity dropdown filter, it only filters activity items by blog posts and not groupblog posts.

This commit allows both types of blog posts to be filtered in the activity loop when the "Posts" activity filter is selected.

Let me know what you think.

[boonebgorges]

Excellent - thanks, @r-a-y !