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>

templates/devtest/gitea-ui.tmpl

@@ -0,0 +1,12 @@
+{{template "base/head" .}}
+<div class="page-content devtest">
+	<div>
+		<gitea-origin-url data-url="test/url"></gitea-origin-url>
+		<gitea-origin-url data-url="/test/url"></gitea-origin-url>
+	</div>
+	<div>
+		<span data-tooltip-content="test tooltip">text with tooltip</span>
+	</div>
+	{{template "shared/combomarkdowneditor" .}}
+</div>
+{{template "base/footer" .}}

templates/devtest/list.tmpl

@@ -0,0 +1,5 @@
+<ul>
+	{{range .SubNames}}
+	<li><a href="{{AppSubUrl}}/devtest/{{.}}">{{.}}</a></li>
+	{{end}}
+</ul>