homepage: fix title not shown without background image

[goapunk]

fixes #803

[philli-m]

as @extend overwrites any other styling you add if it clashes with the extended styling I would more inclined to add the .ds-hero-image__overlayclass to the styling above so have

.ds-hero-no-image,
 ds-hero-image__overlay {
        margin-bottom: 5 * $spacer !important;
        text-align: center;
 }

 // overwrite of herounit text box
 .ds-hero-image__overlay { ect.

but as it's more a coding syntax choice so easy either way

[philli-m]

if you want to change the styling go for it but it's not essential so also feel free to merge without refactor

[philli-m]

very nice! :)