[1.7.x] Fixed #22680 -- I/O operation on closed file.

This patch is two-fold; first it ensure that Django does close everything in
request.FILES at the end of the request and secondly the storage system should
no longer close any files during save, it's up to the caller to handle that --
or let Django close the files at the end of the request.

Backport of e2efc896 from master.