Track timestamp of last reply by journalist to a source

[eloquence]

In the SecureDrop Client, we'd like to be able to show a timestamp that indicates the last action in a given conversation (a journalist reply, a source message or document, etc.). This is freedomofpress/securedrop-client#538. Right now we're only tracking the last source action (last_updated field in the sources table).

We should not alter the behavior of the last_updated field, as it is used for notifications about source activity. We should instead add a separate field to track the timestamp of the most recent journalist reply.

Acceptance Criteria

Given that I am a journalist interacting with a source
When I successfully respond to the source through the SecureDrop Client or the Journalist Interface
Then the timestamp of my most recent response to the source is set or updated in the database

Given that I am developer accessing the Journalist Interface API
When I query for information about a given source using the API
Then the timestamp of the most recent journalist reply (if any) is available to me in the API response

[ninavizz]

^ Might (or should?) the above be edited to reflect the caveat the When as "When my response is successfully sent to the Source"? Even though I know communications never actually leave the server...

[eloquence]

Thanks, tweaked language slightly.

[sssoleileraaa]

I no longer have access to any of the old docs, so I was wondering what happened to the ux examples around showing the last activity timestamp (vs last source activity timestamp) and a per-message/reply timestamps?