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/actions/runner_edit.tmpl

@@ -95,7 +95,7 @@
 		</table>
 		{{template "base/paginate" .}}
 	</div>
-	<div class="ui small modal" id="runner-delete-modal">
+	<dialog class="modal-dialog" id="runner-delete-modal">
 		<div class="header">
 			{{svg "octicon-trash"}}
 			{{ctx.Locale.Tr "actions.runners.delete_runner_header"}}
@@ -104,5 +104,5 @@
 			<p>{{ctx.Locale.Tr "actions.runners.delete_runner_notice"}}</p>
 		</div>
 		{{template "base/modal_actions_confirm" .}}
-	</div>
+	</dialog>
 </div>