Add web app manifest file

CHDataManagement/Model/Content+Generation.swift

@@ -175,6 +175,9 @@ extension Content {
         if let css {
             result.insert(.css(file: css, order: HeaderElement.defaultCssFileOrder))
         }
+        if let manifest = settings.pages.manifestFile {
+            result.insert(.manifest(manifest))
+        }
         return result
     }