User import/export migration ability

[jospoortvliet]

On large instances it is wise to use federation for the best scalability. At that point, it makes sense to move very active users to a more powerful instance or spit a overloaded instance in two. This requires the ability to export user data from one and import them in another instance; or a direct migration path.

[MorrisJobke]

Duplicate of icewind1991/migration#1