LPS-49802 Comments API: transfer all Message Boards references from Discussion Taglib JSP files

Hi @sergiogonzalez!

This pull decouples the Discussion Taglib presentation from the Message Boards implementation, as explained in: https://issues.liferay.com/browse/LPS-49801

The Comments API, which had started small (https://issues.liferay.com/browse/LPS-44577, https://issues.liferay.com/browse/LPS-44582) is now quite a bit larger. The improvements are mostly the stuff needed to query and render a comments section, extracted away from the scriptlets in the Discussion Taglib's JSP files. I'm looking forward to your feedback concerning names and overall design of the new interfaces. They roughly follow their counterparts in Message Boards but there are a couple of essential differences. @marcellustavares has reviewed and I've already incorporated his changes.

Please note this is a fairly large pull request and perhaps you'll prefer to not send it forward as it is. I'm sending it in full so you can have a good understanding of the final intended result, however as you send back your insights I can incorporate them and resend in smaller chunks to improve chances of faster approval upstream. Of course if you assess it's already small enough to push forward, please do :-)

All refactorings were backed by JSP unit tests, asserting that the final rendered HTML is the same as before for all points of extraction and their variations. The tests stayed green before and after the changes. Unfortunately the portal project doesn't yet have a home for JSP unit tests but you can find them here:
 https://github.com/arboliveira/liferay-portal-jsp-unit-test/blob/LPS-49802/src/test/java/com/liferay/jsp/taglib/ui/discussion/TaglibUIDiscussion_page_jsp_Test.java

@rotty3000 With these changes, by the time we extract the upcoming Message Boards OSGi module it should be possible to perform a clean stop on it, in which case Blog entries, Wiki pages and the like should graciously hide their comments section until the module is active again or a replacement is installed.

Also, we can now introduce the new Comments API interfaces to the Mobile team so @brunofarache can start thinking about what remote endpoints they'd like to build and expose. As the back-end behavior is now available to use outside the JSP, they may even attempt a first mobile UI design of a comments section modeled after the classic web one.