Group provisioning: delete only groups created by the plugin

[bergerar]

When group provisioning is enabled and a user looses access to a group, the plugin will also remove the group from the user in Nextcloud. This is great for groups created by the plugin.
But the plugin also removes all the groups not assinged by the plugin from a user when he logins.

I think the plugin should differentiate between groups created by the plugin and manually created groups.

We would need this change for an integration with Hitobito as a OAuth provider. The related issue is hitobito/hitobito_jubla#74 (it is in german).

I already worked on some Nextcloud plugins, so I could make a pull request for this issue.