Refactor markdown editor and use it for milestone description editor (#32688)

Refactor markdown editor to clarify its "preview" behavior and remove
jQuery code.

Close #15045

---------

Co-authored-by: silverwind <me@silverwind.io>

web_src/css/editor/combomarkdowneditor.css

@@ -96,6 +96,11 @@
   font-size: 0.85em;
 }
 
+.combo-markdown-editor .ui.tab.markup[data-tab-panel="markdown-previewer"] {
+  border-bottom: 1px solid var(--color-secondary);
+  padding-bottom: 1rem;
+}
+
 text-expander {
   display: block;
   position: relative;