Save the original filename alongside the upload url

[rashfael]

Uploading files on venueless works largely by uploading to a generic endpoint and receiving an internal url, which the frontend then writes into a string field.
When that happens, the original file name is lost, and the user only sees something like /media/pub/sample/ac7cfb98-a1f2-4756-a9d2-ffa6d2ef6312.HnU4zTvRk7VH.png, which is not useful to glance any information on what the file may have contained.

Somehow preserving the original filename would be nice, at least for the admin config, but care should be taken to not leak filenames to every attendee or for things like avatar images.