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

Hi @sergiogonzalez!

This is exactly the same as [my original pull request](https://github.com/brianchandotcom/liferay-portal/pull/18819), only arguments and variable names were changed from `message*` to `comment*` so we enforce the convention that the Comments API should conceal the underlying Message Boards implementation.

Please read [my comments here](https://github.com/brianchandotcom/liferay-portal/pull/18819#issuecomment-44440759) explaining our plans for fleshing out the Comments API in the next upcoming small pull requests, as well as introducing proper models and services further down the road.