`::class` is available since PHP 5.5 (5.5 is a minimal requirement by Phorge): https://www.php.net/manual/en/language.oop5.basic.php#language.oop5.basic.class.class
It makes finding code using IDEs easier; see discussion in D25500.
Thus replace all string return values with returning the `::class` constant instead, with one exception: 'PhabricatorSettingApplication' in `PhabricatorUserPreferencesSearchEngine.php` does not exist and makes arc lint fail so this string remained unchanged.
Also note that two occurrences were wrapped in `pht()` for reasons I do not know.
List of functions whose return value get updated in this code change:
* getApplicationClassName()
* getAdapterApplicationClass()
* getDatasourceApplicationClass()
* getEditorApplicationClass()
* getEngineApplicationClass()
* getPHIDTypeApplicationClass()
* getQueryApplicationClass()