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/ImagePaste.js

@@ -25,6 +25,10 @@ function clipboardPastedImages(e) {
   return files;
 }
 
+function triggerEditorContentChanged(target) {
+  target.dispatchEvent(new CustomEvent('ce-editor-content-changed', {bubbles: true}));
+}
+
 class TextareaEditor {
   constructor(editor) {
     this.editor = editor;
@@ -38,6 +42,7 @@ class TextareaEditor {
     editor.selectionStart = startPos;
     editor.selectionEnd = startPos + value.length;
     editor.focus();
+    triggerEditorContentChanged(editor);
   }
 
   replacePlaceholder(oldVal, newVal) {
@@ -54,6 +59,7 @@ class TextareaEditor {
     }
     editor.selectionStart = editor.selectionEnd;
     editor.focus();
+    triggerEditorContentChanged(editor);
   }
 }
 
@@ -70,6 +76,7 @@ class CodeMirrorEditor {
     endPoint.ch = startPoint.ch + value.length;
     editor.setSelection(startPoint, endPoint);
     editor.focus();
+    triggerEditorContentChanged(editor.getTextArea());
   }
 
   replacePlaceholder(oldVal, newVal) {
@@ -84,6 +91,7 @@ class CodeMirrorEditor {
     endPoint.ch += newVal.length;
     editor.setSelection(endPoint, endPoint);
     editor.focus();
+    triggerEditorContentChanged(editor.getTextArea());
   }
 }