External files, improve page generation

CHDataManagement/Storage/Storage.swift

@@ -379,6 +379,18 @@ final class Storage {
         return try readExistingFile(at: path)
     }
 
+    // MARK: External file list
+
+    private let externalFileListName = "external-files.json"
+
+    func loadExternalFileList() throws -> [String] {
+        try read(at: externalFileListName, defaultValue: [])
+    }
+
+    func save(externalFileList: [String]) throws {
+        try writeIfChanged(externalFileList.sorted(), to: externalFileListName)
+    }
+
     // MARK: Website data
 
     private let settingsDataFileName: String = "settings.json"