UnitTest around changelog, set correct time of page source

ChangeLog constructor will check the date of current page/media file to detect external edits via getCurrentRevisionInfo()

add needed type hints for phpunit8

This will break a lot of plugin tests, but can't be avoided

split changelog classes into their own namespace

The remaining functions in inc/changelog.php should be moved into a
utility class.

rename unit tests for getRevisionAround. Fix revs order as well.