Messages: Reformat new message notifications.

This commit changes the single message notification link to go
directly to the message thread in question and not the message inbox.

When the message thread is viewed, new messages are marked as read.

Fixes #5479

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8202 cdf35c40-ae34-48e0-9cc9-0c9da1808c22