Do not expose hidden users' names in file uploads.

This change SHA1 sums the username.
Script to migrate screenshot names from username form to sha1 form
Fix script which migrates filenames away from username paths.

Back when, image uploads were not validated... Now they are alongside
stripping them of any potentially identifying metadata.

This change stops Imagemagick barfing on old, non-image uploads. It
lists the filename to give the opportunity to validate (and delete) them.
Reverting to original Media Resultset, dissociate username from filename
Merge pull request #318 from duckduckgo/hidden-usernames-in-screenshots

Do not expose hidden users' names in file uploads.