[FEATURE] Assign one user to multiple clients

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Is your feature request related to a problem? Please describe. We have some users that should be in different Clients at the same time. We have an external IT company which works for multiple clients and the same user keeps track of the projects for different clients.

Another reason is, we separated our company in clients for marketing, support, development. Some users should be in multiple clients this way also.

Describe the solution you'd like Allow to select more then one client for a user.