[design-system]: Allow root elements to be further adjusted using styled-components

[SeverS]

Every component in design system must have className prop, in order to be able to customize it via styled components.

For example:
I want to add a padding for apps widget card. Using styled-components i can write

const AppsCard = styled(AppsWidgetCard)`
  padding: 0.5em;
`

In the background styled method will create a new css class name and pass it as className prop to AppsWidgetCard and that className should be passed further to the root element.

[SeverS]

solved by #200