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>

---------

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

routers/web/devtest/devtest.go

@@ -0,0 +1,35 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package devtest
+
+import (
+	"net/http"
+	"path"
+	"strings"
+
+	"code.gitea.io/gitea/modules/base"
+	"code.gitea.io/gitea/modules/context"
+	"code.gitea.io/gitea/modules/templates"
+)
+
+// List all devtest templates, they will be used for e2e tests for the UI components
+func List(ctx *context.Context) {
+	templateNames := templates.GetTemplateAssetNames()
+	var subNames []string
+	const prefix = "templates/devtest/"
+	for _, tmplName := range templateNames {
+		if strings.HasPrefix(tmplName, prefix) {
+			subName := strings.TrimSuffix(strings.TrimPrefix(tmplName, prefix), ".tmpl")
+			if subName != "list" {
+				subNames = append(subNames, subName)
+			}
+		}
+	}
+	ctx.Data["SubNames"] = subNames
+	ctx.HTML(http.StatusOK, "devtest/list")
+}
+
+func Tmpl(ctx *context.Context) {
+	ctx.HTML(http.StatusOK, base.TplName("devtest"+path.Clean("/"+ctx.Params("sub"))))
+}