LPS-44579 TrackbackAction: introduce tests to document current behavior

https://issues.liferay.com/browse/LPS-44579
LPS-44579 Used Java 1.7 API; fix it to be compatible with Java 1.6

https://issues.liferay.com/browse/LPS-44579
LPS-44579 SF
LPS-44579 Message Boards API usage moved to its own specific method

https://issues.liferay.com/browse/LPS-44579
LPS-44579 Comment creation logic moved to its own specific class

https://issues.liferay.com/browse/LPS-44579
LPS-44579 Comment creation test moved to its own specific class

https://issues.liferay.com/browse/LPS-44579
LPS-44579 Writing 'when..thenReturn' reads better than 'doReturn..when'

https://issues.liferay.com/browse/LPS-44579
LPS-44579 Make validation logic decoupled from error reporting format

https://issues.liferay.com/browse/LPS-44579
LPS-44579 Rely on the Request for logic, Response is for reporting only

https://issues.liferay.com/browse/LPS-44579
LPS-44579 Validation logic moved to its own specific method

https://issues.liferay.com/browse/LPS-44579
LPS-44579 Don't hand over the ActionRequest to comment creation logic

https://issues.liferay.com/browse/LPS-44579
LPS-44579 Trackback creation logic moved to its own specific method

https://issues.liferay.com/browse/LPS-44579
LPS-44579 Trackback creation logic moved to its own specific class

https://issues.liferay.com/browse/LPS-44579
LPS-44579 Trackback creation test moved to its own specific class

https://issues.liferay.com/browse/LPS-44579
LPS-44579 Make TrackbackValidationException a PortalException
LPS-44579 Add Function interface to portal, replaced third party API dependency
LPS-44579 SF
LPS-44579 Use singular as in the rest of the places
LPS-44579 Validate methods should return void. I extracted the logic to obtain the blog entry outside of the validate method because I think that it's important enough to be outside, because the ActionUtil.getEntry method is the reason why we can later obtain the entry from the request, and we shouldn't hide this logic inside a validate method
LPS-44579 Include the logic in one single method because otherwise it may create confusion because one of them send error/success but the other doesn't. Renamed from addTrackbackComment to addTrackback