Add dynamic cell sizes

This PR 
- makes the size of the image cells dynamic so that it works on different device types. 
- makes it possible to switch off the custom transition when presenting the `UIImagePickerController`, because it does not work properly when the photo chooser is presented from a modal with presentation style `UIModalPresentationFormSheet`
- removes a method that is never called and a method that is obsolete
I would prefer using the ivar