Set constant FE_USER_LOGGED_IN

[bytehead]

Request from payment provider to system/modules/isotope/postsale.php:

ErrorException: Use of undefined constant FE_USER_LOGGED_IN - assumed 'FE_USER_LOGGED_IN' (this will throw an Error in a future version of PHP)
#14 /vendor/isotope/isotope-core/system/modules/isotope/library/Isotope/Model/ProductCollection/Cart.php(55): handleError
#13 /vendor/isotope/isotope-core/system/modules/isotope/library/Isotope/Model/ProductCollection/Cart.php(55): getBillingAddress
#12 /vendor/isotope/isotope-core/system/modules/isotope/library/Isotope/Isotope.php(273): calculatePrice
#11 /vendor/isotope/isotope-core/system/modules/isotope/library/Isotope/Model/ProductPrice.php(90): getOriginalAmount
#10 /vendor/isotope/isotope-core/system/modules/isotope/library/Isotope/Collection/ProductPrice.php(95): getOriginalAmount
#9 /templates/iso_collection_cart.html5(65): include
#8 /system/modules/core/library/Contao/BaseTemplate.php(88): parse
#7 /system/modules/core/library/Contao/Template.php(277): parse
#6 /system/modules/core/classes/FrontendTemplate.php(46): parse
#5 /vendor/isotope/isotope-core/system/modules/isotope/library/Isotope/Model/ProductCollection/Order.php(480): getNotificationTokens
#4 /vendor/isotope/isotope-core/system/modules/isotope/library/Isotope/Model/ProductCollection/Order.php(303): updateOrderStatus
#3 /vendor/isotope/isotope-core/system/modules/isotope/library/Isotope/Model/ProductCollection/Order.php(222): checkout
#2 /vendor/isotope/isotope-core/system/modules/isotope/library/Isotope/Model/Payment/PSP.php(125): processPostsale
#1 postsale.php(175): run
#0 postsale.php(245): null