The directory we were previously saving backups to on the external SD
card is actually deleted upon app uninstall and/or clearing the app's
data. There's also no reliable way to write to the root of an external
SD card (that isn't comically inconvenient), so for now it's safer if we
just move back to getting the regular 'ol standard external storage
directory (which is likely internal storage, despite its name).
Fixes#7845
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.
