Cleanups

A few refactorings. 
* took in @joshuaadickerson findings in https://github.com/elkarte/Elkarte/pull/427 controllers using topics participation.
* reworked messagesSince() and countMessagesSince() to be consistent. They are working on the same thing, just one does count, the other gets the messages ids. Updated them to be more reusable, both now accept $only_approved, and a switch to include the (current) message in the result. ($include_current)