WoltLab Suite: Preserve known output classes for user options

[dtdesign]

Currently all output classes for user options are discarded during an import because a missing output class would brick certain parts of the software. While this make sense for unknown output classes, there is little to no reason to discard output classes that are actually known to the software such as wcf\system\option\user\NewlineToBreakUserOptionOutput. We should check if the class is known to the system and only discard unknown classes.