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/web.go

@@ -27,6 +27,7 @@ import (
 	"code.gitea.io/gitea/modules/web/routing"
 	"code.gitea.io/gitea/routers/web/admin"
 	"code.gitea.io/gitea/routers/web/auth"
+	"code.gitea.io/gitea/routers/web/devtest"
 	"code.gitea.io/gitea/routers/web/events"
 	"code.gitea.io/gitea/routers/web/explore"
 	"code.gitea.io/gitea/routers/web/feed"
@@ -1491,6 +1492,12 @@ func RegisterRoutes(m *web.Route) {
 	if setting.API.EnableSwagger {
 		m.Get("/swagger.v1.json", SwaggerV1Json)
 	}
+
+	if !setting.IsProd {
+		m.Any("/devtest", devtest.List)
+		m.Any("/devtest/{sub}", devtest.Tmpl)
+	}
+
 	m.NotFound(func(w http.ResponseWriter, req *http.Request) {
 		ctx := context.GetContext(req)
 		ctx.NotFound("", nil)