Implement background image

• Add functionality to bakground-image selector in competition and slide
• Give a warning that ratio should be 16/9
• Display background image (background -> images -> text, in z-axis)

changed milestone to %Vecka 17

added Client Development labels

created branch 144-implement-background-image to address this issue

mentioned in merge request !109 (merged)

closed with merge request !109 (merged)