Enhancement: Turn placeholders in OcModal into descriptionMessages

We have dropped the support for placeholders in the `<oc-text-input>` component.
As this component is used inside OcModal, this change also drops placeholder support 
and adds the possibilty to instead make use of a descriptionMessage (which is only being 
display if no inputError is present).

https://github.com/owncloud/owncloud-design-system/pull/1172
