Replace some manual class name strings with ::class keyword
Summary:
Replace classname strings in `isClassInstalled()` parameter value with the corresponding `::class` keyword.
This will enable static code analysis to throw "Class not found" if such a class did not exist.
`::class` is available since PHP 5.5: https://www.php.net/manual/en/language.oop5.basic.php#language.oop5.basic.class.class
`::class` returns the fully qualified classname including namespace, however we do not use PHP namespaces except in imported external libraries.
See similar previous patches.
Refs T15158
Test Plan: Browse random Phorge pages?
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T15158
Differential Revision: https://we.phorge.it/D26383