Support printing conversations

[eloquence]

Description

"Print conversation" feature to print all messages and replies in a source conversation.

The MVP could reside in the menu of source-level actions, and route a plain-text file generated from all messages and replies through the existing print workflow.

How will this impact SecureDrop users?

Journalist users have reported that sometimes, messages contain the only substantive content in a submission. It is therefore important that we support both exporting (#550) and printing entire conversations.

How would this affect the SecureDrop Workstation threat model?

This does not impact the existing threat model, which already assumes that printers may be attached and detached to print files.

User Stories

As a journalist, I want to print a conversation, so that I can treat it like any other submission.

[eloquence]

(Note that we could also support printing at the level of individual messages and replies, but that provides a level of granularity that is probably far less commonly needed.)