LPS-47303 Comments API: transfer all Message Boards references from BaseIndexer

Hi @brianchandotcom,

As requested in https://github.com/brianchandotcom/liferay-portal/pull/19059#issuecomment-45258463, this is just a resend of the remaining commit to drop class references, now rebased against the latest master with all conflicts fixed.

Thanks!

cc @rotty3000