There was an issue where we were backing up group receipts and attachments
that were for expiring messages (which are already excluded from the backup).
This commit excludes these items from the backup, and for backups made
before this change, this commit also deletes these invalid entries at
the end of the restore process.
We also do a little database migration to cleanup any bad state that may
have been imported in the past.
First, FTS index contents do not need to be exported. They will be recreated naturally.
Second, we can't export the secret FTS tables, or SQLite will think it's corrupted.
