Add og:image from the first image in a page; fix Hansaplatz post image

layouts/_partials/head/extra.html

@@ -0,0 +1,40 @@
+{{- /*
+Open Graph image for link previews. The theme's head/meta.html writes og:image only for the
+"image" param; without it, this uses the first image in the page's Markdown, scaled down to at
+most 1200 pixels wide, with its alt text as og:image:alt.
+*/ -}}
+{{- if not ($.Param "image") -}}
+  {{- with findRESubmatch `!\[([^\]]*)\]\(\s*<?([^\s)>]+)` .RawContent 1 -}}
+    {{- $alt := index . 0 1 -}}
+    {{- $dest := index . 0 2 -}}
+    {{- $url := "" -}}
+    {{- $width := 0 -}}
+    {{- $height := 0 -}}
+    {{- $resource := false -}}
+    {{- if findRE `^https?://` $dest -}}
+      {{- $url = $dest -}}
+    {{- else -}}
+      {{- $resource = or ($.Resources.Get (strings.TrimPrefix "./" $dest)) (resources.Get $dest) -}}
+    {{- end -}}
+    {{- with $resource -}}
+      {{- $image := . -}}
+      {{- if and (eq .ResourceType "image") (ne .MediaType.SubType "svg") -}}
+        {{- if gt .Width 1200 }}{{ $image = .Resize "1200x" }}{{ end -}}
+        {{- $width = $image.Width -}}
+        {{- $height = $image.Height -}}
+      {{- end -}}
+      {{- $url = $image.Permalink -}}
+    {{- end -}}
+    {{- with $url }}
+<meta property="og:image" content="{{ . }}">
+{{- with $width }}
+<meta property="og:image:width" content="{{ . }}">
+<meta property="og:image:height" content="{{ $height }}">
+{{- end }}
+{{- with $alt }}
+<meta property="og:image:alt" content="{{ . }}">
+{{- end }}
+<meta name="twitter:card" content="summary_large_image">
+    {{- end -}}
+  {{- end -}}
+{{- end -}}