Copy bbPress reply trash and spam logic.

This is a copy and paste of changes from:
* r4674 - https://bbpress.trac.wordpress.org/changeset/4674
* r4675 - https://bbpress.trac.wordpress.org/changeset/4675
Copy bbPress topic conditional checks from r4778:
https://bbpress.trac.wordpress.org/changeset/4778
Make bbPress reply log message more generic.
Remove cron check on 'admin_init' action.

'init' already fires in the admin area, so this is redundant and could
cause unneeded IMAP connections to the inbox.
Add duplicate reply check for legacy forums.
Be extra cautious when initializing new stdClass.
Add email subject line to 'no body' bounce-back email.
For non-RBE emails, add a line above each email message stating that you
should not reply to this email.

This is to prevent the recipient from replying to non-RBE emails, which
will not get parsed by RBE.

Note: This line will not show up on activation emails and when an email is
sent from the WP admin dashboard.

All other emails will add the line.

eg. friendship request emails, group member promotion emails, and any other
email that is sent from the frontend.

This can be disabled with the 'bp_rbe_show_non_rbe_notice' filter if needed.