Add title suffix, extract HTML elements

Fix header calls

WebsiteGenerator/Templates/Filled/LocalizedSiteTemplate.swift

@@ -142,5 +142,20 @@ struct LocalizedSiteTemplate {
         return fullDateFormatter.string(from: start)
             .insert(" - \(day.string(from: end))", beforeLast: ",")
     }
+
+    func makeHeaderContent(page: Element, metadata: Element.LocalizedMetadata, language: String) -> [HeaderKey : String] {
+        let backText = page.backLinkText(for: language)
+
+        var content = [HeaderKey : String]()
+        content[.backLink] = makeBackLink(text: backText, language: language)
+        if let suffix = metadata.titleSuffix {
+            content[.title] = factory.html.make(title: metadata.title, suffix: suffix)
+        } else {
+            content[.title] = metadata.title
+        }
+        content[.subtitle] = metadata.subtitle
+        content[.date] = makeDateString(start: page.date, end: page.endDate)
+        return content
+    }
 }