Fine tune markdown editor toolbar (#24046)

1. Remove unnecessary `btn-link` `muted` classes
* Link is link, button is button, I can't see a real requirement to make
a button like a link.
* If anyone insists, please help to show me real example from modern
frameworks / websites, how and why they do so.
    * No need to duplicate a lot of class names on similar elements
* Declare styles clearly, for example, `markdown-toolbar` itself should
have `display: flex`, but not use `gt-df` to overwrite the `display:
block`.
2. Remove unnecessary `role` attribute
    * https://github.com/github/markdown-toolbar-element/issues/70
* The `markdown-toolbar-element` does want to add `role=button`, but
there is a bug.
* So we do the similar thing as upstream does (add the role by JS),
until they fix their bugs.
3. Indent `markdown-switch-easymde` (before it doesn't have a proper
indent)

Screenshot:

![image](https://user-images.githubusercontent.com/2114189/231090912-f6ba01cb-d0eb-40ad-bf8c-ffc597d9a778.png)

web_src/css/editor-markdown.css

@@ -4,7 +4,8 @@
 
 .combo-markdown-editor markdown-toolbar {
   cursor: default;
-  display: block;
+  display: flex;
+  align-items: center;
   padding-bottom: 10px;
 }
 
@@ -12,9 +13,21 @@
   display: flex;
 }
 
+.combo-markdown-editor .markdown-toolbar-group:last-child {
+  flex: 1;
+  justify-content: flex-end;
+}
+
 .combo-markdown-editor .markdown-toolbar-button {
+  border: none;
+  background: none;
   user-select: none;
   padding: 5px;
+  cursor: pointer;
+}
+
+.combo-markdown-editor .markdown-toolbar-button:hover {
+  color: var(--color-primary);
 }
 
 .ui.form .combo-markdown-editor textarea.markdown-text-editor,