Add activity meta when recording a blog post or blog comment.

This commit:
- Adds the `post_title` and `post_url` information as activity meta when recording a blog post or blog comment into the activity table.
- Updates the `post_title` and `post_url` activity meta whenever a blog post is updated.
- Adds a unit test.

This addresses issues with showing the correct blog post information in the
activity loop for blog post and blog comment activity items.

Props r-a-y, DJPaul, boonebgorges.

Fixes #5609.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8944 cdf35c40-ae34-48e0-9cc9-0c9da1808c22