Improve AJAX link and modal confirm dialog (#25210)

Clarify the "link-action" behavior:

>  // A "link-action" can post AJAX request to its "data-url"
> // Then the browser is redirect to: the "redirect" in response, or
"data-redirect" attribute, or current URL by reloading.

And enhance the "link-action" to support showing a modal dialog for
confirm. A similar general approach could also help PRs like
https://github.com/go-gitea/gitea/pull/22344#discussion_r1062883436

> // If the "link-action" has "data-modal-confirm(-html)" attribute, a
confirm modal dialog will be shown before taking action.


And a lot of duplicate code can be removed now. A good framework design
can help to avoid code copying&pasting.

---------

Co-authored-by: silverwind <me@silverwind.io>

tests/integration/branches_test.go

@@ -34,7 +34,7 @@ func TestDeleteBranch(t *testing.T) {
 func TestUndoDeleteBranch(t *testing.T) {
 	onGiteaRun(t, func(t *testing.T, u *url.URL) {
 		deleteBranch(t)
-		htmlDoc, name := branchAction(t, ".undo-button")
+		htmlDoc, name := branchAction(t, ".restore-branch-button")
 		assert.Contains(t,
 			htmlDoc.doc.Find(".ui.positive.message").Text(),
 			translation.NewLocale("en-US").Tr("repo.branch.restore_success", name),