markdown: Fix bullet list indentation bug.

Previously, the frontend-only javascript processor didn't show the same
output as the backend, resulting in wrong output shown in drafts.

Fixes zulip#10966

static/third/marked/lib/marked.js

@@ -269,7 +269,11 @@ Lexer.prototype.token = function(src, top, bq) {
 
     // list
     if (cap = this.rules.list.exec(src)) {
-      src = src.substring(cap[0].length);
+
+      // Reduces the odd number of spaces
+      // to even.
+      cap[0] = cap[0].replace(/^((  )*) ?/gm, "$1");
+      src = src.substring(cap[0].length + 1);
       bull = cap[2];
 
       this.tokens.push({