Check integrity of .zip files
Check contents of uploaded zip and reject it if it contains any
non-XML files.

For now, we are performing only a superficial check for file
extensions. It might make sense to extend this check to also verify
MIME-types of individual files.
Catch BadZipFile in zip integrity validator
Rename corrupted_files variable in zip integrity validator to
"corrupted_file" to reflect the fact that ZipFile.testzip() returns
the first corrupted file it finds, not a list of files
Rename validate_zip_format to validate_extension to reflect the fact
that we allow .xml files as well