Support markdown editor for issue template (#24400)

Fixes #24398

Task:
- [x] Reusing "textarea" like GitHub seems more friendly to users.
- [x] ^V image pasting and file uploading handling.

<details><summary>screenshots</summary>


![image](https://user-images.githubusercontent.com/18380374/235418877-00090552-ebda-411c-8e39-b47246bc8746.png)

![image](https://user-images.githubusercontent.com/18380374/235419073-dc33cad7-7626-4bce-9161-eb205c7384b5.png)
Display only one markdown editor:

![image](https://user-images.githubusercontent.com/18380374/235419098-ee21386d-2b2d-432e-bdb2-18646cc031e7.png)
Support file upload and ^V image pasting

![image](https://user-images.githubusercontent.com/18380374/235419364-7b390fa4-da56-437d-b55e-3847fbc049e7.png)

</details>

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>

web_src/js/features/comp/QuickSubmit.js

@@ -6,7 +6,9 @@ export function handleGlobalEnterQuickSubmit(target) {
   if ($form.length) {
     // here use the event to trigger the submit event (instead of calling `submit()` method directly)
     // otherwise the `areYouSure` handler won't be executed, then there will be an annoying "confirm to leave" dialog
-    $form.trigger('submit');
+    if ($form[0].checkValidity()) {
+      $form.trigger('submit');
+    }
   } else {
     // if no form, then the editor is for an AJAX request, dispatch an event to the target, let the target's event handler to do the AJAX request.
     // the 'ce-' prefix means this is a CustomEvent