Separate print, USB export related actions into their own classes

[redshiftzero]

Closes #33, breaks up the SDExport class, we now have:

• SDExport (same name) which contains submission archive related information and helper methods, like the nice Metadata object that was already on master
• *Action objects, one for each type of export we support, which contain methods/attributes related to the export action in question. Each export action must implement run().

Two things that could be followups would be:

• see if we can make SDExport.exit_gracefully more standalone: I ended up not making a utils.py as the exit_gracefully method is used almost everywhere, and part of its cleanup is removing the submission's tmpdir. So I'd either need to pass tmpdir around everywhere, which is messy, or just leave it as a method on SDExport, which is what I elected to do for now. The fact this is used so widely is also what complicated removing the submission from the test actions.
• combining usb-test and disk-test: I didn't do that here as I wanted to keep the inter-VM interface the same (i.e. no changes are needed in securedrop-client to merge this) but I think this would make sense to do.

Since I'm moving around the furniture, feel free to let me know if you don't like where things are @emkll & @creviera :-)

Test plan

I have tested the following with my HP LaserJet and a USB drive:

• Ensure you can still USB export successfully
• Ensure you can print a document successfully

[sssoleileraaa]

Two things that could be followups would be:

• see if we can make SDExport.exit_gracefully more standalone: I ended up not making a utils.py as the exit_gracefully method is used almost everywhere, and part of its cleanup is removing the submission's tmpdir. So I'd either need to pass tmpdir around everywhere, which is messy, or just leave it as a method on SDExport, which is what I elected to do for now. The fact this is used so widely is also what complicated removing the submission from the test actions.

I agree that the preflight check Actions and PrintTestPageAction should not require a submission object. It seems like it makes more sense for there to be a Printer object with printer_uri etc that you can pass to printer Actions, and a DiskDrive object that you pass to disk transfer actions. I think it would make sense to create a followup discussion issue.

• combining usb-test and disk-test: I didn't do that here as I wanted to keep the inter-VM interface the same (i.e. no changes are needed in securedrop-client to merge this) but I think this would make sense to do.

I agree with this followup.

✔️ I made sure I could still print a file from a LaserJet Pro M404n printer and transfer a file to a usb disk drive.

[redshiftzero]

thanks for the comments! agreed all around, added two commits (and did a quick retest of the USB export... i mean Disk export 😇)

[sssoleileraaa]

lgtm