Make wiki default branch name changable (#29603)

Fix #29000
Fix #28685
Fix #18568

Related: #27497

And by the way fix #24036, add a Cancel button there (one line)

templates/repo/wiki/pages.tmpl

@@ -10,6 +10,7 @@
 				{{end}}
 			</span>
 		</h2>
+		{{if .IsRepositoryAdmin}}<div>{{ctx.Locale.Tr "repo.default_branch"}}: {{.Repository.DefaultWikiBranch}}</div>{{end}}
 		<table class="ui table wiki-pages-list">
 			<tbody>
 				{{range .Pages}}