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.

templates/repo/view_file.tmpl

@@ -42,6 +42,11 @@
 				</div>
 				<a download href="{{$.RawFileLink}}"><span class="btn-octicon" data-tooltip-content="{{.locale.Tr "repo.download_file"}}">{{svg "octicon-download"}}</span></a>
 				<a id="copy-content" class="btn-octicon {{if not .CanCopyContent}} disabled{{end}}"{{if or .IsImageFile (and .HasSourceRenderedToggle (not .IsDisplayingSource))}} data-link="{{$.RawFileLink}}"{{end}} data-tooltip-content="{{if .CanCopyContent}}{{.locale.Tr "copy_content"}}{{else}}{{.locale.Tr "copy_type_unsupported"}}{{end}}">{{svg "octicon-copy" 14}}</a>
+				{{if .EnableFeed}}
+				<a class="btn-octicon" href="{{$.FeedURL}}/rss/{{$.BranchNameSubURL}}{{range $i, $v := .TreeNames}}/{{$v}}{{end}}">
+					{{svg "octicon-rss" 14}}
+				</a>
+				{{end}}
 				{{if .Repository.CanEnableEditor}}
 					{{if .CanEditFile}}
 						<a href="{{.RepoLink}}/_edit/{{PathEscapeSegments .BranchName}}/{{PathEscapeSegments .TreePath}}"><span class="btn-octicon" data-tooltip-content="{{.EditFileTooltip}}">{{svg "octicon-pencil"}}</span></a>