[WIP] delete tags when user is deleted

:construction: 

Delete tags and relations when a user is deleted.

Fixes https://github.com/owncloud/core/issues/12816

This is tricky because I cannot use ITags because it's based on type.
I cannot use the TagMapper alone as it's only the tags, not the relations.
So... trying to sneak this in... feels a bit convoluted.

@icewind1991 @ockham @LukasReschke @Raydiation