Consider implications of owncloud username renaming on federated sharing

[tomneedham]

As part of #29503 ownCloud is considering what is used as the ownCloud user_id/username/login/id throughout ownCloud, and also how it is exposed to external systems. Whilst migrations are possible internally, knowing the full context of the change, external systems require notification of the change, and if possible a migration to keep the functionality consistent.

We can use this ticket to discuss implications and possible solutions.

Ongoing & impacted by the outcome of #29503 ...

[PVince81]

no update since a while, moving to backlog