Phase out usage of sync_flag file

[sssoleileraaa]

Description

During sync, a file called sync_flag is created in the ~/.securedrop_client directory on sd-app in order to track timestamps for each metadata sync. To clean up files the client creates on the fs, we could instead track these in svs.sqlite.

It also might be a good time to reconsider the trade-offs for tracking last-sync timestamps between client instances. Right now, if you log on in offline mode, you can see the last time you synced with the server because we record these timestamps. If we wanted to minimize the data we store in the sd-app db, we could instead show the last-sync timestamps only within a single instance by tracking last-sync in-memory only.