Theme compat content cannot be filtered

[ddean]

Calls to bp_buffer_template_part hooked to the bp_replace_the_content filter echo content rather than returning it.

Content is immediately displayed instead of being passed along to later functions hooking bp_replace_the_content.

[ddean]

I'm attaching a patch for Groups as an example, but it's like this for the Activity, Blogs, Forums, Groups, and Members components.

[r-a-y]

Good catch. We should do this for all components like you stated, ddean.

[ddean]

Expanded to include all components

[r-a-y]

Thanks for the refresh, ddean.

[r-a-y]

In 7386:

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.