Replace __CLASS__ constant with self::class keyword
Summary:
`::class` is available since PHP 5.5.
* https://www.php.net/manual/en/language.constants.magic.php
* https://www.php.net/manual/en/language.oop5.basic.php#language.oop5.basic.class.class
* https://wiki.php.net/rfc/class_name_scalars#considerations
`::class` returns the fully qualified classname including namespace, however we do not use PHP namespaces except in imported external libraries untouched here.
(Also, I ideally like to grep for 1 term when trying to find the same stuff.)
Refs T15158
Test Plan: Browse random Phorge pages?
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: mainframe98, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T15158
Differential Revision: https://we.phorge.it/D26324