Get user by ID or username
Get a specific user by ID provided the user is within this requesting user's organisation or the organisation's immediate child organisations.
`id` may be one of:
* The numeric ID of the user, e.g. "123". This is the top-level `id` object returned by each item in `GET /api/v1/user`
* Username (email), e.g. "exampleuser@org.de". This is the top-level `username` object returned by each item in `GET /api/v1/user`