Change: Remove icon prop from buttons and use css grid

We've removed the prop which was responsible for displaying icons in the button component.
To add an icon into the button, it needs to be included in the slot together with the text.
We've added css grid into the button to ensure that all child items of the button will have a predefined gutter between them.
We've removed all UIKit button styles from the button component.

https://github.com/owncloud/owncloud-design-system/pull/418
https://github.com/owncloud/owncloud-design-system/pull/865
