LPS-46461 Comments API: transfer all Message Boards references from Pingbacks

Hi @marcellustavares!

Another one! After everything https://issues.liferay.com/browse/LPS-46460 changed, I also had to remake https://github.com/marcellustavares/liferay-portal/pull/253 from scratch.

If all is well please forward to @sergiogonzalez copying @rotty3000. Thanks!