feat: Delete messages from a chatmail server immediately by default (#5805) (#5840)

I.e. treat `DeleteServerAfter == None` as "delete at once". But when a backup is exported, set
`DeleteServerAfter` to 0 so that the server decides when to delete messages, in order not to break
the multi-device case. Even if a backup is not aimed for deploying more devices, `DeleteServerAfter`
must be set to 0, otherwise the backup is half-useful because after a restoration the user wouldn't
see new messages deleted by the device after the backup was done. But if the user explicitly set
`DeleteServerAfter`, don't change it when exporting a backup. Anyway even for non-chatmail case the
app should warn the user before a backup export if they have `DeleteServerAfter` enabled.

Also do the same after a backup import. While this isn't reliable as we can crash in between, this
is a problem only for old backups, new backups already have `DeleteServerAfter` set if necessary.

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Co-authored-by: Hocuri <hocuri@gmx.de>

python/src/deltachat/testplugin.py

@@ -525,6 +525,7 @@ class ACFactory:
         configdict.setdefault("mvbox_move", False)
         configdict.setdefault("sentbox_watch", False)
         configdict.setdefault("sync_msgs", False)
+        configdict.setdefault("delete_server_after", 0)
         ac.update_config(configdict)
         self._acsetup._account2config[ac] = configdict
         self._preconfigure_key(ac, configdict["addr"])