LPS-46436 Comments API: absorb "addComment" from TrackbackComments

https://issues.liferay.com/browse/LPS-46436
LPS-46436 Comments API: absorb "deleteComment" from LinkbackConsumerUtil

https://issues.liferay.com/browse/LPS-46436
LPS-46436 Linkbacks: Drop every last reference to MBMessageLocalService

https://issues.liferay.com/browse/LPS-46436
LPS-46436 Linkbacks: stop using a static class for singleton purposes

https://issues.liferay.com/browse/LPS-46436
LPS-46436 LinkbackConsumer singleton: make it modular and injectable

https://issues.liferay.com/browse/LPS-46436
LPS-46436 Inject the Comments API instead of passing it as an argument

https://issues.liferay.com/browse/LPS-46436
LPS-46436 Linkbacks: Drop every last reference to MBMessage

https://issues.liferay.com/browse/LPS-46436
LPS-46436 Leave as it was for consistency
LPS-46436 Let's model this after com.liferay.portal.kernel.workflow. I'm not calling it a CommentService, because we'll think ServiceBuilder, so let's use *Manager. Also, call it CommentManager and not Comments or else when you have a list of Comment, you won't know what it is by the var name
LPS-46436 Use full var name
LPS-46436 Use full var name, imagine if we had a MBEntry how confusing this would be if we did not namespace it
LPS-46436 Forgot test
LPS-46436 SF
LPS-46436 SF
LPS-46436 Avoid constructors
LPS-46436 SF
LPS-46436 Avoid constructors
LPS-46436 SF
LPS-46436 Auto SF