Add Stolperstein to the list of memorial options

**Use case** <!-- Provide a clear and concise description of *your use case* and what you thus think is missing, and why. --> A Stolperstein is a common memorial for victims of the holocaust. There are currently over 50000 objects already tagged with memorial=stolperstein and I think it would be handy to have this type present in the app. **Proposed Solution** <!-- There may be several different solutions for your idea or use case, add your deliberations about it here to help sort out the best one Add screenshots or mockups to visualize your idea, if applicable. --> Add Stolperstein as a type of memorial to be selected in the app, when classifying one https://wiki.openstreetmap.org/wiki/Tag:memorial=stolperstein