Template tags and filters from app in COMMON_EXTRA_APPS no longer available

Hello,

I have a custom app installed in COMMON_EXTRA_APPS, which makes custom template tags and filters available through a file in its templatetags subdirectory. This used to work in earlier versions of Mayan (3.4.9), but has stopped working with 4.4.2. I believe the cause is the creation of a custom django Engine in apps/templating/classes.py, which results in djangos usual loading mechanism for template tags from apps being circumvented.

The reason for the custom Engine seems to be to make the new mayan math filters / template tags available through the builtins argument. I would suggest using the builtins option of the django.template.backends.django.DjangoTemplates instead: https://docs.djangoproject.com/en/3.2/topics/templates/#module-django.template.backends.django (scroll down)

This could be added in settings/base.py to the OPTIONS like so:

TEMPLATES = [
    {
        'BACKEND': 'django.template.backends.django.DjangoTemplates',
        'DIRS': [],
        'OPTIONS': {
            'builtins': [
                'mathfilters.templatetags.mathfilters',
                'mayan.apps.templating.templatetags.templating_tags'
            ],
            'context_processors': [
                'django.template.context_processors.debug',
                'django.template.context_processors.i18n',
                'django.template.context_processors.request',
                'django.contrib.auth.context_processors.auth',
                'django.contrib.messages.context_processors.messages'
            ],
            'loaders': [
                'django.template.loaders.filesystem.Loader',
                'django.template.loaders.app_directories.Loader'
            ]
        }
    }
]

I have not tested this yet, since I don't have a proper development environment right now.