Add GraphQL end point that lists assigned clients for a user

[fermion]

This will be used to drive the view of staffplans where projects are grouped by the client. tl;dr:

  assignedClients(userId optional, if passed will filter for that user) {
    projects(active filter default to true?) {
      project stuff
    } 
  }

[fermion]

We went away from this idea, instead grouping things in the client before rendering.