LPS-46436 Comments API: transfer all Message Boards references from Trackbacks and Linkbacks

Hi @sergiogonzalez!

This PR is for the Blogs subsystem. It contains the full set of changes to extract MBMessage references away from Trackbacks and Linkbacks. This introduces the first proto-version of the Comments abstraction class.

As @rotty3000 remarked, this is just a minimal abstraction with no intention of adding any improvements other than decoupling Comments from the Portal core. Also, there are more changesets on the way that need this in before. (Pingbacks, JSPs, wire the Comments impl via Spring.)

Please review and send back to me with your fixes. I'll incorporate them and resend as small PRs.

cc @marcellustavares (this version already incorporates the fixes from your first review)

_Reminders:_

All caps `URL` is encouraged in method names and variables
https://github.com/liferay/liferay-portal/commit/212ab9ba77c930feafc68af52fc39f1be9fd7b13#diff-90f7e9d80d74a01a4071a27502998e0bL200