[chore] Use marked v4

Copied from #2

Actually updating to v4.0.10
The only relevant breaking change: https://github.com/markedjs/marked/releases/tag/v4.0.0 
is that the default export is removed: `({marked} = require('marked'))`

Parens are required to make it not a SyntaxError.

Only other change is that the docs: https://marked.js.org/using_advanced
recommend using sanitize on the output instead of in the options

Author: Steven nguyen

lib/helpers.js

@@ -515,21 +515,17 @@ let domPurify = null;
 
 export function renderMarkdown(md) {
   if (marked === null) {
-    marked = require('marked');
+    ({marked} = require('marked'));
 
     if (domPurify === null) {
       const createDOMPurify = require('dompurify');
       domPurify = createDOMPurify();
     }
 
-    marked.setOptions({
-      silent: true,
-      sanitize: true,
-      sanitizer: html => domPurify.sanitize(html),
-    });
+    marked.setOptions({silent: true});
   }
 
-  return marked(md);
+  return domPurify.sanitize(marked(md));
 }
 
 export const GHOST_USER = {