Fix broken image at top of city page
Fix broken image at top of city page

The top of https://codefor.de/en/heidelberg/ contains a link to `/wahlsalons`, which has an `img` child element pointing to `img/wahlsalons_banner.svg`, which does not display in my browser. When I follow this relative link to `https://codefor.de/en/heidelberg/img/wahlsalons_banner.svg`, I land on the same page, which is not an image. https://codefor.de//img/wahlsalons_banner.svg does exist and is an image. The link should thus be absolute.
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