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/new.tmpl

@@ -36,9 +36,8 @@
 			</div>
 			<div class="divider"></div>
 			<div class="text right">
-				<button class="ui primary button">
-					{{ctx.Locale.Tr "repo.wiki.save_page"}}
-				</button>
+				<a class="ui basic cancel button" href="{{.Link}}">{{ctx.Locale.Tr "cancel"}}</a>
+				<button class="ui primary button">{{ctx.Locale.Tr "repo.wiki.save_page"}}</button>
 			</div>
 		</form>
 	</div>

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}}