Add HEIC codec to gallery/image viewer

[SchlennoB]

Currently submitted images and photos in HEIC format are not displayed in the gallery and in a submissions detailed view. Since iPhones have a significant share in the mobile market it might be reasonable to add a codec that allows to view submitted images in HEIC format in the backend. Especially in the context of self administered forms when users might not even be aware of the format their photos are saved with.