MDL-39359 - mod/assign: Allow zipped folder to be uploaded for returning feedback files

The "Upload multiple feedback files in a zip" feature is convenient, but requires that
the files sit directly within the zip file. As the files will most likely be extracted
to a folder after downloading, it's quite reasonable to expect teachers to zip the
folder up rather than selecting all files and zipping them.  This patch allows for a
zipped folder to be uploaded; if the only file found after extracting is a directory,
the contents of the directory are used instead of the results of the extraction.