Backup: zipped messages.json, flat attachments dir

Backup creates, in a target directory:
  - An attachments folder, with all attachments, each named for their
    parent message's id - a GUID. If there is more than one attachment
    in a given message,  each attachment beyond the first will end with
    '-N', zero-indexed.
  - A file named messages.zip. It contains exactly what went to disk in
    the original export code, but zipped up.

Export is now only 'light,' and in this new messages.zip format.

Import supports both the new format and the old format. If the target
directory has a messages.zip file, we'll treat it as the new format.

Next up: Encrypting attachments and the messages.zip!