Added "edit listing" function

The listings are now editable, however if you do not upload a new image,
the original is removed. I still need to add some logic to test if a new
image is uploaded, and if not, keep the original.
Added "edit listing" feature

The listings are now editable, however, if you do not upload a new
image, it will erase the existing one. I will still need to add some
logic to determine if a file has been uploaded, and if not, keep the
existing one.