Establish user/group links via users endpoints

[tcompa]

Extracting from #1731:

• Add the possibility of linking users to group also within POST/PATCH users endpoints (while it's now only possible via groups endpoints).

We currently have PATCH /auth/group/{id}/ with payload new_user_ids:list[int].
We should also include a new request-body attribute new_group_ids in PATCH /auth/users/{id}/.