Add 'Copy as Markdown' action

Remove smart quotes from Markdown export, closes #5303

shared/utils/MarkdownHelper.ts

@@ -0,0 +1,29 @@
+interface DocumentInterface {
+  emoji?: string | null;
+  title: string;
+  text: string;
+}
+
+export default class MarkdownHelper {
+  /**
+   * Returns the document as cleaned Markdown for export.
+   *
+   * @param document The document or revision to convert
+   * @returns The document title and content as a Markdown string
+   */
+  static toMarkdown(document: DocumentInterface) {
+    const text = document.text
+      .replace(/\n\\\n/g, "\n\n")
+      .replace(/“/g, '"')
+      .replace(/”/g, '"')
+      .replace(/‘/g, "'")
+      .replace(/’/g, "'")
+      .trim();
+
+    const title = `${document.emoji ? document.emoji + " " : ""}${
+      document.title
+    }`;
+
+    return `# ${title}\n\n${text}`;
+  }
+}