Bugfix: OcSpinner make ariaLabel prop optional

The OcSpinner component had a required property that caused console errors. 
Since we're not always using it in a case where a label is required, we've 
made it optional but strongly recommend to use it unless the element is 
wrapped in/equiped with a `aria-hidden="true"` attribute.

https://github.com/owncloud/owncloud-design-system/pull/1251
