Consider optionally attributing replies to sources in Source Interface

[eloquence]

Reply attribution (which journalist sent a given reply) will be visible to journalists in the SecureDrop Client when freedomofpress/securedrop-client#1142 lands. We should consider to also make it visible to sources in the Source Interface. This almost certainly would need to be optional (possibly per-reply) as journalists may sometimes need to respond to problematic submissions where attribution is not desirable.

User research evidence

No clear evidence from journalists or sources yet that this is desirable, though we know that some sources may have a preference for reaching a specific journalist (cf. #1355 and #1550), and therefore may also want to know that they're speaking with that journalist.

User story

As a source, I want to know who I'm talking to, so I'm not disclosing information to someone I don't want to disclose it to.

Previous mockups

From 2017 by @ninavizz, for discussion purposes only:

Refactored from #1592 filed by @psivesely