Add an ITEM_UNSEEN event type, to allow marking a source as unread

[micahflee]

References freedomofpress/securedrop-client#2853.

In order to mark sources as unread, we need to be able to mark items as unseen. This PR adds the new ITEM_UNSEEN event type, and implements it.

As per this comment, when you mark an item as unread, it marks it as not downloaded and deletes all seen rows associated with it. This means that it's possible that a journalist can download a file from a source then mark it as unseen, deleting the evidence that they've downloaded it.

I think though allowing journalists to do this is better UX, and we're not designing the server-side database to be an audit trail. (If we were, we'd make a separate audit log table.) So I think this is fine, but open to discussion.

Test plan

Checklist

This change accounts for:

• any required additional documentation
• any necessary AppArmor changes (added or removed application files)
• any impact on new SecureDrop installs and upgrades
• our dependency update policy