Show uploaded RegistrationField images

[matthijskooijman]

When you have an IMAGE-type RegistrationField, you can upload an image during registration. However, when you go back to a previously uploaded image, it shows only the filename, not the image itself, in a less-than-ideal UI:

This should show the image itself, and probably a nicer UI to clear a previously uploaded image.

Similarly, the image should probably be shown in the admin somehow.

One challenge is that these files cannot be served directly by the static webserver, because some authentication needs to be applied to them.