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/admin/emails/list.tmpl

@@ -76,7 +76,7 @@
 
 		{{template "base/paginate" .}}
 
-		<div class="ui g-modal-confirm modal" id="change-email-modal">
+		<dialog class="modal-dialog" id="change-email-modal">
 			<div class="header">
 				{{ctx.Locale.Tr "admin.emails.change_email_header"}}
 			</div>
@@ -95,7 +95,7 @@
 
 				{{template "base/modal_actions_confirm" .}}
 			</form>
-		</div>
+		</dialog>
 	</div>
 
 {{template "admin/layout_footer" .}}