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/repo/diff/new_review.tmpl

@@ -7,14 +7,19 @@
 	<div class="review-box-panel tippy-target">
 		<div class="ui segment">
 			<form class="ui form" action="{{.Link}}/reviews/submit" method="post">
-			{{.CsrfTokenHtml}}
+				{{.CsrfTokenHtml}}
 				<input type="hidden" name="commit_id" value="{{.AfterCommitID}}">
 				<div class="header gt-df gt-ac gt-pb-3">
 					<div class="gt-f1">{{$.locale.Tr "repo.diff.review.header"}}</div>
 					<a class="muted close gt-px-3">{{svg "octicon-x" 16}}</a>
 				</div>
 				<div class="ui field">
-					<textarea name="content" tabindex="0" rows="2" placeholder="{{$.locale.Tr "repo.diff.review.placeholder"}}"></textarea>
+					{{template "shared/combomarkdowneditor" (dict
+						"locale" $.locale
+						"TextareaName" "content"
+						"TextareaPlaceholder" ($.locale.Tr "repo.diff.review.placeholder")
+						"DropzoneParentContainer" "form"
+					)}}
 				</div>
 				{{if .IsAttachmentEnabled}}
 					<div class="field">