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.
When params doesn't contain auth_key, `params[:auth_key].present? && ...` is evaluated to `false` while `params[:auth_key].present && ...` is evaluated to `nil`.
It's a bad situation where `@member` could be both `nil` and `false`, IMO.