add support for user_migration

[jospoortvliet]

Is your feature request related to a problem? Please describe.
When exporting your user settings for migration, currently the activities are not coming along.

It would be great if what happened to your files & other activities, plus notification settings, would be there on your new Nextcloud server.

[miaulalala]

Hm, there are two issues I see

• if an activity happens on a file the exporting user that wasn't done by that user, that would need to be filtered out as the other user may or may not exist on the instance the data is being migrated to
• What happens if apps created activities, and those new apps don't exist on the new instance