Add RSS Feeds for branches and files (#22719)

Fix #22228 adding RSS feeds for branches and files. 

RSS feeds are accessed through:

* [gitea]/src/branch/{branch}.rss
* [gitea]/src/branch/{branch}/{file_name}.rss

No changes have been made to the UI to expose the feed urls for branches
and files.

routers/web/feed/render.go

@@ -0,0 +1,22 @@
+// Copyright 2022 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package feed
+
+import (
+	model "code.gitea.io/gitea/models/repo"
+	"code.gitea.io/gitea/modules/context"
+)
+
+// RenderBranchFeed render format for branch or file
+func RenderBranchFeed(ctx *context.Context) {
+	_, _, showFeedType := GetFeedType(ctx.Params(":reponame"), ctx.Req)
+	var renderer func(ctx *context.Context, repo *model.Repository, formatType string)
+	switch {
+	case ctx.Repo.TreePath == "":
+		renderer = ShowBranchFeed
+	case ctx.Repo.TreePath != "":
+		renderer = ShowFileFeed
+	}
+	renderer(ctx, ctx.Repo.Repository, showFeedType)
+}