Updated extract packages code

Removed references to old Extract Packages from maildir, metadata and
submissionDocumentation paths.  Updated maildir file identification to do
extraction as well, like standard path.

Extract contents checks if the identified format is NULL and doesn't try to
use that format.  Django model for identified files table matches DB in
allowing NULL fields.
Removed deprecated identifyFilesByExtension

Removed script and database entries referencing identifyFilesByExtension,
since it's been replaced by the new FPR identification scripts.  Removed
also deprecated treatment of maildir files from extract Maildir attachments.