Allow authentication by api_key

Currently the authentication by api_key is only allowed in RpcController.
It would be useful if other API (e.g., /api/pin/all) can be accessed by api_key.
I think `Object#present?` is better than `Object#presence` here.
Because the left expression is "guard condition", so boolean value is more natural for the return value of left expression.
Thus `params[:auth_key].present?` looks readable.