Implement soft delete (disable) of journalists accounts

[heartsucker]

Description

In order to support #3925 (which in turn supports freedomofpress/securedrop-client#76), we may want to do a soft delete of journalists in the database. Or, we may want to support both hard and soft deletes (to allow for eventual clean up of the DB).

Consider this scenario:

• Source submits an initial inquiry to SD
• Journalist A has some number of exchanges with Source
• Journalist A is removed from SD (for some reason)
• Journalist B syncs the client with the last sync being before Source submitted to SD
• Journalist B opens the conversation with Source
• Journalist B is unable to determine who sent the messages to Source

This could be solved in #3925 by storing the username or display_name or a journalist in the table instead of the journalist's id or uuid. However, this feels wrong since we would want to show the display_name in the client if it exists, and that may be a mutable field.

User Stories

As a journalist who jumps into a conversation with a source after the another journalist has conversed with them and quit or gotten fired, I want to know who was talking to the source and not just see null.

[eloquence]

This is currently solved in the SecureDrop Client by showing such users with a "deleted" icon. See related discussion in #5467. This is still a valid issue as we may indeed want both: a way to preserve the association with fully deleted users, and a way to disable users without deleting them.