Refactor find forks and fix possible bugs that weak permissions check (#32528)

- Move models/GetForks to services/FindForks
- Add doer as a parameter of FindForks to check permissions
- Slight performance optimization for get forks API with batch loading
of repository units
- Add tests for forking repository to organizations

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/repo/forks.tmpl

@@ -5,12 +5,14 @@
 		<h2 class="ui dividing header">
 			{{ctx.Locale.Tr "repo.forks"}}
 		</h2>
+		<div class="flex-list">
 		{{range .Forks}}
-			<div class="tw-flex tw-items-center tw-py-2">
-				<span class="tw-mr-1">{{ctx.AvatarUtils.Avatar .Owner}}</span>
-				<a href="{{.Owner.HomeLink}}">{{.Owner.Name}}</a> / <a href="{{.Link}}">{{.Name}}</a>
+			<div class="flex-item tw-border-0 repo-fork-item">
+				<span>{{ctx.AvatarUtils.Avatar .Owner}}</span>
+				<span><a href="{{.Owner.HomeLink}}">{{.Owner.Name}}</a> / <a href="{{.Link}}">{{.Name}}</a></span>
 			</div>
 		{{end}}
+		</div>
 	</div>
 
 	{{template "base/paginate" .}}