Make skeleton compatible with objectstore stable7

:construction: Waiting for an ocsid

@icewind1991 @DeepDiver1975 @PVince81 fixes https://github.com/owncloud/core/issues/9985

I doubt we will be backporting https://github.com/owncloud/core/pull/10697 for stable7, so I will log an error when the order of apps is wrong, causing the encryption to receive the post login hook after the skeleton.

@karlitschek I need an ocsid because I moved the skeleton generation related code to a self contained app. How do I get one?