LPS-44579 TrackbackAction: introduce tests to document current behavior

TrackbackAction is now modularized in three smaller classes with specific responsibilities.

- `TrackbackAction` continues to be the entry point and presentation boundary, dealing with ActionRequest and ActionResponse. Validations that are tightly coupled to ActionRequest and HttpServletRequest also remain here.

- `Trackbacks` now holds only the actual trackback creation logic. The input values for this class are pure model objects, with no more references to ActionRequest or HttpServletRequest at this level.

- `TrackbackComments` now encapsulates and hides all Message Boards API usages. This will later become the target of https://issues.liferay.com/browse/LPS-44577.

Each of these classes has its own unit test, checking for each level's different responsibilities.