SecureDrop Client does not create seen records for users who have never sent a reply

[eloquence]

Steps to reproduce

1. Log in via Journalist Interface as a user who has never sent a reply (user A)
2. Submit a new message or file as a source
3. Download it via the JI
4. Now log in as a different user via the SecureDrop Client (user B)

Note: The same behavior can be observed by using two SecureDrop Clients with separate client directories.

Expected behavior

Upon sync, the client database should record that user A has seen the message or file, in the seen_messages or seen_files tables.

Actual behavior

The seen data is never added (#1197 will ensure that we still show a correct global seen state for the source, by interpreting the legacy downloaded / is_read flag Boolean; once that PR lands, there are no obvious UX implications from this issue).

Notes

This is because we rely on reply attribution before we create users other than the logged in user, and we don't add seen records if we don't have a user to associate them with. #1157 is expected to resolve this and other issues related to user synchronization.

[eloquence]

Per my comment on #1197 (comment), since there are no immediate UX implications once that PR lands, and we get pristine seen data on each sync, I don't think this is a release blocker for 0.4.0.