Implement users.get RPC function

[mathemancer]

We need to implement a users.get RPC function to replace the GET /api/ui/v0/users/{userId}/ endpoint.

The function signature should be:

@rpc_method(name='users.get')
@http_basic_auth_login_required
@handle_rpc_exceptions
def get(
        *,
        user_id: int,
) -> UserInfo:

See the wiki for more context.