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

Hi @rotty3000!

This PR is for the **Search** subsystem. Together with [this one](https://github.com/sergiogonzalez/liferay-portal/pull/1440) they wipe out every single instanceof-like reference to `MBMessage.class` from Search core classes, extracting them out to the MBMessageIndexer -- ready to modularize away.

Please send back to me or forward to @sergiogonzalez at your discretion.

(Already has the contributions from @marcellustavares review.)