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>

web_src/css/editor-markdown.css

@@ -0,0 +1,25 @@
+.combo-markdown-editor {
+  width: 100%;
+}
+
+.combo-markdown-editor markdown-toolbar {
+  cursor: default;
+  display: block;
+  padding-bottom: 10px;
+}
+
+.combo-markdown-editor .markdown-toolbar-group {
+  display: inline-block;
+}
+
+.combo-markdown-editor .markdown-toolbar-button {
+  user-select: none;
+  padding: 5px;
+  cursor: pointer;
+}
+
+.combo-markdown-editor .markdown-text-editor {
+  display: block;
+  width: 100%;
+  height: 200px;
+}

web_src/css/editor.css

@@ -13,7 +13,6 @@
 }
 
 .editor-toolbar {
-  max-width: calc(100vw - 80px);
   border-color: var(--color-secondary);
 }
 

web_src/css/index.css

@@ -29,6 +29,7 @@
 @import "./form.css";
 @import "./repository.css";
 @import "./editor.css";
+@import "./editor-markdown.css";
 @import "./organization.css";
 @import "./user.css";
 @import "./dashboard.css";

web_src/css/repository.css

@@ -2116,10 +2116,6 @@
   height: 48px;
 }
 
-.repository.wiki.new .ui.attached.tabular.menu.previewtabs {
-  margin-bottom: 15px;
-}
-
 .repository.wiki.view > .markup {
   padding: 15px 30px;
 }

web_src/css/review.css

@@ -248,6 +248,11 @@ a.blob-excerpt:hover {
   }
 }
 
+.review-box-panel .combo-markdown-editor textarea {
+  width: 730px;
+  max-width: calc(100vw - 70px);
+}
+
 #review-box {
   position: relative;
 }