add markup to thread create and topic edit

views/common/bbcode_help.etlua

@@ -0,0 +1,11 @@
+<details>
+  <summary>Supported babycode tags</summary>
+  <ul>
+    <li>[b]<b>bold</b>[/b]</li>
+    <li>[i]<i>italic</i>[/i]</li>
+    <li>[s]<del>strikethrough</del>[/s]</li>
+    <li>[url=https://example.com]<a href="https://example.com">labeled URL</a>[/url]</li>
+    <li>[url]<a href="https://unlabeled-url.example.com">https://unlabeled-url.example.com</a>[/url]</li>
+    <li>[code]<code>code block</code>[/code]</li>
+  </ul>
+</details>