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>

routers/web/misc/markup.go

@@ -15,24 +15,6 @@ import (
 
 // Markup render markup document to HTML
 func Markup(ctx *context.Context) {
-	// swagger:operation POST /markup miscellaneous renderMarkup
-	// ---
-	// summary: Render a markup document as HTML
-	// parameters:
-	// - name: body
-	//   in: body
-	//   schema:
-	//     "$ref": "#/definitions/MarkupOption"
-	// consumes:
-	// - application/json
-	// produces:
-	//     - text/html
-	// responses:
-	//   "200":
-	//     "$ref": "#/responses/MarkupRender"
-	//   "422":
-	//     "$ref": "#/responses/validationError"
-
 	form := web.GetForm(ctx).(*api.MarkupOption)
 
 	if ctx.HasAPIError() {