Create proposal: secure document pipeline

[jskinne3]

• Dangerzone integration
• Signal integration
• export+print RPC overhaul

These issues are at least tangentially related:

• [securedrop-export] Detect if sufficient storage space is available prior to exporting files #1739
• Support viewing of html files #2008
• Proposal: Consider publishing optional keyring+sources package on apt repo for 3rd party tooling #2142
• Remove sd-devices from SDW securedrop-workstation#1503
  The issues @deeplow links to below are more central

[deeplow]

Thanks for getting this started. These topics are spread among individual issues, mostly on the workstation repo.

Here are the ones I could find:

• Implement export with file "sanitization" (content disarm/reconstruct) securedrop-workstation#862 (Dangerzone integration)
  • Also discussed in the context on priting, but it should also apply there
    Research alternatives to current print architecture securedrop-workstation#564 (comment)
• Implement alternative to qvm-open-in-vm (sd-app → sd-devices) securedrop-workstation#671. There is also:
  • an issue to formalize these semantics, which is also good to have in mind formalize qrexec RPC semantics securedrop-workstation#1175
  • a historical (internal) document discussing how to approach this problem
• Signal integration securedrop-workstation#609

But I think it makes sense to have a proposal with these together since they are all in practice all connected and inter-dependent.