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

Hi @marcellustavares!

(see: https://issues.liferay.com/browse/LPS-49801)

This pull decouples the Discussion Taglib from the Message Boards module. There is quite a bit more of Comments API now, mostly the stuff needed for rendering a comments section, extracted away from the scriptlets in the Discussion Taglib's JSP files. Please provide your feedback concerning names and overall design of the new interfaces.

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

After these changes it should be possible to stop cleanly the upcoming Message Boards OSGi module, in which case Blog entries, Wiki pages and the like should graciously hide their comments section until the module is active again.

Also, we can show the new Comments API interfaces to the Mobile team so they can start thinking about what remote endpoints they'd like to build and expose. They may even attempt a first design of a comments section for the mobile UI, modeled after the classic one, as the back-end behavior is now available outside the JSP.