Finalize export metadata format

[eloquence]

#259 implemented a basic, manual export workflow and specified (in the README) a metadata format for exports:

.
├── metadata.json
└── export_data
    ├── file-to-export-1.txt
    ├── file-to-export-2.pdf
    ├── file-to-export-3.doc
    [...]

metadata.json example:

{
  "encryption-method": "luks"
  "encryption-key": "Your encryption passhrase goes here"
}

Before we implement export support in the client (freedomofpress/securedrop-client#21) we should be sure that this is the format specification we want to go with. For example, is there other information we want to include in the metadata or folder structure (e.g., source name, date/time of export)?

[eloquence]

It may be worth comparing this approach with the ZIP folder structure generated by downloads from the Journalist Interface, which looks like this:

all
├── recessive accreditation
│   ├── 1_2019-07-07
│   │   └── 1-recessive_accreditation-msg.gpg
│   └── 2_2019-07-07
│       └── 2-recessive_accreditation-msg.gpg
└── surviving authentication
    ├── 1_2019-07-07
    │   └── 1-surviving_authentication-doc.gz.gpg
    └── 2_2019-07-07
        └── 2-surviving_authentication-msg.gpg

The date used for the folders is the "last updated" date for a given source (https://github.com/freedomofpress/securedrop/blob/develop/securedrop/store.py#L137).

[eloquence]

See also the related issue of handling filename conflicts: #270

[emkll]

A draft of the metadata archive format specification can be found in the following document https://docs.google.com/document/d/1qoQMMGjB9d9mHelZg_lncgiMjjB1BxIvZot_2Y6KjqM for discussion as part of the 7/24-8/7 sprint.

[sssoleileraaa]

@emkll I gave the google doc a first pass review and left a few comments and some suggestions.

[redshiftzero]

also dropped some comments inline, otherwise LGTM!

[emkll]

Thanks @creviera and @redshiftzero for the comments/suggestions, they have all been addressed/accepted, and they should now be consistent with the changes in freedomofpress/securedrop-export#5 . I think this should now be sufficient to close the ticket. If anyone disagrees, please feel free to repoen/comment.