Introduce GitHub markdown editor, keep EasyMDE as fallback (#23876)

The first step of the plan

* #23290

Thanks to @silverwind for the first try in #15394 . Close #10729 and a
lot of related issues.

The EasyMDE is not removed, now it works as a fallback, users can switch
between these two editors.

Editor list:

* Issue / PR comment
* Issue / PR comment edit
* Issue / PR comment quote reply
* PR diff view, inline comment
* PR diff view, inline comment edit
* PR diff view, inline comment quote reply
* Release editor
* Wiki editor

Some editors have attached dropzone

Screenshots:

<details>


![image](https://user-images.githubusercontent.com/2114189/229363558-7e44dcd4-fb6d-48a0-92f8-bd12f57bb0a0.png)


![image](https://user-images.githubusercontent.com/2114189/229363566-781489c8-5306-4347-9714-d71af5d5b0b1.png)


![image](https://user-images.githubusercontent.com/2114189/229363771-1717bf5c-0f2a-4fc2-ba84-4f5b2a343a11.png)


![image](https://user-images.githubusercontent.com/2114189/229363793-ad362d0f-a045-47bd-8f9d-05a9a842bb39.png)

</details>

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

templates/repo/issue/fields/textarea.tmpl

@@ -2,5 +2,5 @@
 	{{template "repo/issue/fields/header" .}}
 	{{/* FIXME: preview markdown result */}}
 	{{/* FIXME: required validation for markdown editor */}}
-	<textarea name="form-field-{{.item.ID}}" placeholder="{{.item.Attributes.placeholder}}" class="edit_area {{if .item.Attributes.render}}no-easymde{{end}}" {{if and .item.Validations.required .item.Attributes.render}}required{{end}}>{{.item.Attributes.value}}</textarea>
+	<textarea name="form-field-{{.item.ID}}" placeholder="{{.item.Attributes.placeholder}}" {{if and .item.Validations.required .item.Attributes.render}}required{{end}}>{{.item.Attributes.value}}</textarea>
 </div>