Update from to-markdown to turndown

We got a security alert for a regular expression DoS attack on our used
library `to-markdown`.

After checking `to-markdown` to be maintained or not, it turned out they
renamed the library to `turndown`. So upgrading to `turndown` should fix
this vulnerbility.

References:
https://www.npmjs.com/package/to-markdown
https://github.com/domchristie/turndown/wiki/Migrating-from-to-markdown-to-Turndown
Signed-off-by: Sheogorath <sheogorath@shivering-isles.com>

public/js/index.js

@@ -12,7 +12,7 @@ require('../css/site.css')
 
 require('highlight.js/styles/github-gist.css')
 
-import toMarkdown from 'to-markdown'
+import TurndownService from 'turndown'
 
 import { saveAs } from 'file-saver'
 import randomColor from 'randomcolor'
@@ -1498,7 +1498,12 @@ $('#snippetExportModalConfirm').click(function () {
 })
 
 function parseToEditor (data) {
-  var parsed = toMarkdown(data)
+  var turndownService = new TurndownService({
+    defaultReplacement: function (innerHTML, node) {
+      return node.isBlock ? '\n\n' + node.outerHTML + '\n\n' : node.outerHTML
+    }
+  })
+  var parsed = turndownService.turndown(data)
   if (parsed) { replaceAll(parsed) }
 }