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/package/shared/view.tmpl

@@ -110,7 +110,7 @@
 			<div class="item">
 				{{svg "octicon-trash"}}
 				<a class="show-modal" href data-modal="#delete-package-version-modal">{{ctx.Locale.Tr "packages.settings.delete.version"}}</a>
-				<div class="ui tiny modal" id="delete-package-version-modal">
+				<dialog class="modal-dialog" id="delete-package-version-modal">
 					<div class="header">
 						{{ctx.Locale.Tr "packages.settings.delete.version"}}
 					</div>
@@ -122,7 +122,7 @@
 							{{template "base/modal_actions_confirm" .}}
 						</form>
 					</div>
-				</div>
+				</dialog>
 			</div>
 			{{end}}
 		</div>