Core: merge custom body classes with WordPress' defaults.

Until 1.9, we overrode all the custom body classes that are added to the theme, and replaced them with BuddyPress-specific classes.
With 1.9 in r7663, we manually added a couple of WordPress core classes back to fix compatibility problems with certain themes (notably, Twenty Fourteen). However, body classes added by other plugins were still removed.

This change merges BuddyPress' body classes into those provided by WordPress core, and those from any other plugins.

Fixes #4929 and #5204

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8068 cdf35c40-ae34-48e0-9cc9-0c9da1808c22