Replace CC-BY-SA icon with SVG

[hiddewie]

Part of #850

Split off from #4775

This pull request replaces PNG icons for the CC-BY-SA icons with their minimized SVG variants, to ensure screens with high pixel densities can display the OpenStreetMap website with beautiful graphics.

[tomhughes]

It looks like this one has had the whitespace put back?

By the way you might have noticed I've been fixing up your other branches to squash the whitespace change back into the original edit - we like to try and keep a clean history where we can so it would be better to do that here if you are able. It's also better to rebase on master than merge from it.

[AntonKhorev]

Again this is not on a web page, so concerns similar to emails and #4799 (comment) apply.

[hiddewie]

For Atom feeds SVGs should work: the RFC notes that the feed:rights tag should be a human readable description, either text or HTML. https://validator.w3.org/feed/docs/rfc4287.html#element.rights. HTML with an image should be valid and work in clients that can display images.