User mapping FE and BE - Only show active human users in actions dropdown

To fetch users to show in the actions dropdown, we currently use a GraphQL query that includes all users on the instance:

query searchAllUsersPaginated($search: String!, $after: String = "", $first: Int = null) {
  users(search: $search, after: $after, first: $first) {
    nodes {
      ...User
    }
    pageInfo {
      ...PageInfo
    }
  }
}

In practice, this will also include the placeholder users we just created and if a user tries to select those and reassign, they get the error message "".

This can be confusing for users, so we should filter only users that they can actually reassign for user mapping. This can be done either with an added variable to the query (less changes on frontend) or with a new query.