Have a sticky notification while the export/import is running

[come-nc]

Not sure if we have those kind of notifications?

[PVince81]

@jancborchardt are you aware of such feature request in server ?

[Pytal]

https://github.com/nextcloud/nextcloud-dialogs provides a timeout option which could work although this is not exactly the same as the notification itself being sticky which would be a new feature going by the functionality of the existing interface https://github.com/nextcloud/server/blob/eb5e0a4e511551ae493d7bc3ca77045ba2b9480c/lib/public/Notification/INotification.php

[come-nc]

Proposed plan:

1. Add a notification when the export starts, warning the user he should not use the account while export is running (same for import)
2. When a migrator writes to $output, add the text to the notification if present and possible (Only for normal verbosity, adapt migrators to not be too chatty on this level).
3. If the notification has been closed, open a new one.

And ideally the output lines should also show on the data migration settings page.

Maybe we should use Psr\Log\LoggerInterface instead of Symfony\Component\Console\Output\OutputInterface for migrators to output data so that there are levels for warning and errors?