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

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

web_src/js/index.ts

@@ -83,7 +83,11 @@ import {
   initGlobalButtons,
   initGlobalDeleteButton,
 } from './features/common-button.ts';
-import {initGlobalEnterQuickSubmit, initGlobalFormDirtyLeaveConfirm} from './features/common-form.ts';
+import {
+  initGlobalComboMarkdownEditor,
+  initGlobalEnterQuickSubmit,
+  initGlobalFormDirtyLeaveConfirm,
+} from './features/common-form.ts';
 
 initGiteaFomantic();
 initDirAuto();
@@ -127,6 +131,7 @@ onDomReady(() => {
     initGlobalCopyToClipboardListener,
     initGlobalEnterQuickSubmit,
     initGlobalFormDirtyLeaveConfirm,
+    initGlobalComboMarkdownEditor,
     initGlobalDeleteButton,
 
     initCommonOrganization,