UI for legacy reply flow: replies encrypted and not downloadable

[redshiftzero]

This is a bit of a UX headache, but it's a function of the legacy behavior: from the user's perspective, the following situation is going to be unclear for old messages:

• replies that have been deleted by a source - this will just be a weird discontinuity in the conversation

Note that there's nothing new here, this is a reflection of the current behavior on the web interface, which is confusing for journalists (to resolve this going forward we need freedomofpress/securedrop#3097 implemented over on the server side - edit this is now done)

In the release notes for the client / docs for the workstation, we should include a note about this behavior.

[redshiftzero]

We made the server-side changes over a year ago during the [reply refactor] epic on the server side: freedomofpress/securedrop#3097. Since the journalist will be able to read the conversation history from a year+ ago, I'm closing this as resolved.