refactor: replace Fomantic modal with native dialog (#39846)

Replace Fomantic's hard-forked jQuery modal with native `<dialog>`
elements, advancing https://github.com/go-gitea/gitea/issues/29849. The
UI is visually unchanged. Dialogs open with `show()` over an `inert`
page, so tooltips, popups and toasts mounted on `<body>` stay usable
above them, unlike with `showModal()`
(https://github.com/whatwg/html/issues/9936).

- Remove the Fomantic modal and dimmer, add `showModal` and `hideModal`
- Declare modals as `<dialog class="modal-dialog">`, shown inside a
`.modal-backdrop`, instead of `ui modal` with its unused size classes
- Remove two modals that could never be opened
- Block the page while a modal is open, close the topmost one on Escape
and restore focus to its opener
- Allow stacked modals, use a confirm modal instead of `window.confirm`
for deleting content history
- Hide popups and toasts when a modal opens or closes
- Keep the release notes modal open when generating notes fails

Fixes: https://github.com/go-gitea/gitea/issues/39643
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/shared/secrets/add_list.tmpl

@@ -66,7 +66,7 @@
 </div>
 
 {{/* Add secret dialog */}}
-<div class="ui small modal" id="add-secret-modal">
+<dialog class="modal-dialog" id="add-secret-modal">
 	<div class="header"></div>
 	<form class="ui form form-fetch-action" method="post">
 		<div class="content">
@@ -105,4 +105,4 @@
 		</div>
 		{{template "base/modal_actions_confirm" (dict "ModalButtonTypes" "confirm")}}
 	</form>
-</div>
+</dialog>