Add conversations view

[redshiftzero]

When a user clicks on the source in the sidebar, they should see a conversation view as in @ninavizz's mockups, with the submissions and replies interleaved. There may be replies from multiple journalists.

Unfortunately, SecureDrop server does not currently preserve timestamps on submissions: when a new submission comes in, we update the timestamp on all submissions (we also touch all those files on disk). This is to minimize information stored about sources: we only keep the time of their latest submission.

Fortunately, we do keep an ordering of submissions and replies, via the filename on the submission and reply objects. The integer starting each filename indicates the interaction count for the source. For example, 2-dejected_respondent-msg.gpg is a message that was second in the conversation for dejected respondent.

[ntoll]

OK... this is the next "big" thing I'll implement in terms of UI. I'll make use of @ninavizz's dummy data while building the UI. Once "live" data is running through the app, "it should just work" ™️ ;-)

[redshiftzero]

opened #40 as the followup here, otherwise the UI parts were completed in #34