Instance-wide (global) info banner and maintenance mode (#36571)

The banner allows site operators to communicate important announcements
(e.g., maintenance windows, policy updates, service notices) directly
within the UI.

The maintenance mode only allows admin to access the web UI.

* Fix #2345
* Fix #9618

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/js/features/common-fetch-action.ts

@@ -67,10 +67,15 @@ async function fetchActionDoRequest(actionElem: HTMLElement, url: string, opt: R
 
 async function onFormFetchActionSubmit(formEl: HTMLFormElement, e: SubmitEvent) {
   e.preventDefault();
-  await submitFormFetchAction(formEl, submitEventSubmitter(e));
+  await submitFormFetchAction(formEl, {formSubmitter: submitEventSubmitter(e)});
 }
 
-export async function submitFormFetchAction(formEl: HTMLFormElement, formSubmitter?: HTMLElement) {
+type SubmitFormFetchActionOpts = {
+  formSubmitter?: HTMLElement;
+  formData?: FormData;
+};
+
+export async function submitFormFetchAction(formEl: HTMLFormElement, opts: SubmitFormFetchActionOpts = {}) {
   if (formEl.classList.contains('is-loading')) return;
 
   formEl.classList.add('is-loading');
@@ -80,8 +85,8 @@ export async function submitFormFetchAction(formEl: HTMLFormElement, formSubmitt
 
   const formMethod = formEl.getAttribute('method') || 'get';
   const formActionUrl = formEl.getAttribute('action') || window.location.href;
-  const formData = new FormData(formEl);
-  const [submitterName, submitterValue] = [formSubmitter?.getAttribute('name'), formSubmitter?.getAttribute('value')];
+  const formData = opts.formData ?? new FormData(formEl);
+  const [submitterName, submitterValue] = [opts.formSubmitter?.getAttribute('name'), opts.formSubmitter?.getAttribute('value')];
   if (submitterName) {
     formData.append(submitterName, submitterValue || '');
   }