Introduce bp_get_the_thread_message_css_class()

This centralized function assembles CSS classes for message divs within a
single private message thread. In addition to the existing zebra-striping
classes, classes have been introduced for 'sent-by-[userid]' and 'sent-by-me'.

Fixes #5238

Props sgr33n for an initial patch

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8604 cdf35c40-ae34-48e0-9cc9-0c9da1808c22