use SVG for /favicon.ico if browser asks for it

This makes Chromium use the SVG favicon universally including as the
favicon for non-HTML files.

nginx/nginx.conf

@@ -215,6 +215,9 @@ http {
         }
 
         location = /favicon.ico {
+            if ($http_accept ~ "image/svg\+xml") {
+                rewrite ^ /favicon.svg last;
+            }
             include snippets/security-headers.conf;
             # avoid breaking image hotlinking such as https://github.com/TryGhost/Ghost/issues/12880
             add_header Cross-Origin-Resource-Policy "cross-origin" always;