Securedrop: Better handle conversations between one source and multiple journalists

[redshiftzero]

If Journalist A and Journalist B and both communicating with a source, then with the current architecture of SecureDrop it's quite difficult to handle this conversation because Journalist A might not easily even see that Journalist B has responded to a source if the source logs in, reads the message, and then deletes the reply (as it is requested that they do).

Journalist A _might_ use a feature like #1487 to leave a message for Journalist B letting them know what happened, but if Journalist A and Journalist B are not vigilant about this then Journalist B might not be able to distinguish the "Journalist A forgot to leave a message for Journalist B" scenario from the "Journalist A did not respond to source" scenario. Instead, we should make sure that a feature like #1489 eventually supports multiple journalists interacting with a single source.

[ninavizz]

See annotation number seven

[psivesely]

I think w/ a multi-party secure communication scheme between the new JWs it would even be fine to keep the conversation history so as to avoid the mechanisms you describe. Access could be limited by access controls for journalists (as we've described elsewhere, allowing certain users to assign sources to particular journalists), and data hygiene helpers (something similar to Signal's "disappearing messages" is one idea).

[redshiftzero]

@ninavizz those mockups look really awesome. Definitely something to work towards.

[redshiftzero]

we're handling this over in the client now: https://github.com/freedomofpress/securedrop-client/issues/76