Use seen-by to demarcate "new" messages in client

[rocodes]

Description

In addition to a message thread being seen/unseen, it's helpful for journalists to know which messages are new. We have this functionality in place already (with seen_by), but what's missing is an easy way to visually distinguish a new message from an ongoing conversation. This is a requested feature for higher-traffic instances.

Slack does this with a red bar delineating old from new, but that's in a single-user model (vs a global inbox model).

We could do this for a logged-in journalist by ensuring that, for messages that aren't seen_by them, an additional border decoration, small dot, or other indicator is present.

How will this impact SecureDrop users?

This will make it easier for journalists to catch up on message threads.

How would this affect the SecureDrop Workstation threat model?

No effect, UI only.

User Stories

As a journalist, I want to be able to see at a glance which messages I have not yet read in an ongoing message thread.