Implement toggle prop to decorate Content section components

This allows to add background decorations to the `Content` `ontainer of
the core "Section" container component by simply adding the `decorated`
boolean prop instead of creating a new styled component with a
`position: relative` attribute.

GH-115

src/components/containers/core/Section/Content.jsx

@@ -21,6 +21,7 @@ import CoreContent from "containers/core/Content";
 const Content = styled(CoreContent)`
   padding-top: ${({ compact }) => !compact && "2.5em"};
   padding-bottom: ${({ compact }) => !compact && "2.5em"};
+  position: ${({ decorated }) => decorated && "relative"};
 
   ${({ theme }) => theme.media.tabletLandscape`
     padding-top: ${({ compact }) => !compact && "5em"};