introduce the UserManager interface

                   
|Q            |A  |
|---          |---|
|Bug Fix?     |no |
|New Feature? |yes|
|BC Breaks?   |no |
|Deprecations?|no |
|Tests Pass?  |no |
|Fixed Tickets|   |
|License      |MIT|
                   


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This commit was already pushed but GitHub seems to be stuck with a cache or something.