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.

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

templates/repo/branch/list.tmpl

@@ -165,7 +165,7 @@
 										{{end}}
 										{{if and $.IsWriter (not $.IsMirror) (not $.Repository.IsArchived) (not .IsProtected)}}
 											{{if .IsDeleted}}
-												<button class="btn interact-bg gt-p-3 undo-button" data-url="{{$.Link}}/restore?branch_id={{.DeletedBranch.ID}}&name={{.DeletedBranch.Name}}&page={{$.Page.Paginater.Current}}" data-tooltip-content="{{$.locale.Tr "repo.branch.restore" (.Name)}}">
+												<button class="btn interact-bg gt-p-3 link-action restore-branch-button" data-url="{{$.Link}}/restore?branch_id={{.DeletedBranch.ID}}&name={{.DeletedBranch.Name}}&page={{$.Page.Paginater.Current}}" data-tooltip-content="{{$.locale.Tr "repo.branch.restore" (.Name)}}">
 													<span class="text blue">
 														{{svg "octicon-reply"}}
 													</span>