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/base.css

@@ -328,27 +328,14 @@ progress::-moz-progress-bar {
   user-select: none;
 }
 
-.btn-link {
-  background: none;
-  border: none;
-  color: var(--color-primary);
-}
-
-a:hover,
-.btn-link:hover {
-  text-decoration: underline;
-}
-
 a,
-.ui.breadcrumb a,
-.btn-link {
+.ui.breadcrumb a {
   color: var(--color-primary);
   cursor: pointer;
   text-decoration-skip-ink: all;
 }
 
 a.muted,
-.btn-link.muted,
 .muted-links a {
   color: inherit;
 }
@@ -356,7 +343,6 @@ a.muted,
 a:hover,
 a.muted:hover,
 a.muted:hover [class*="color-text"],
-.btn-link.muted:hover,
 .muted-links a:hover,
 .ui.breadcrumb a:hover {
   color: var(--color-primary);