Enhancement: Make filled primary OcButton use gradient background

We've updated the OcButton to use the gradient background color when used in its `filled` appearance.

https://github.com/owncloud/owncloud-design-system/pull/2036
https://github.com/owncloud/owncloud-design-system/pull/2038
https://github.com/owncloud/owncloud-design-system/issues/1952
