Return contents instead of echoing during theme compat output buffer.

When using the 'bp_replace_the_content' filter, we need to return the
contents instead of echoing.

Thus, this commit changes all bp_buffer_template_part() calls to
return instead of echo.

Fixes #5156.  Props ddean.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7386 cdf35c40-ae34-48e0-9cc9-0c9da1808c22