Improve RSS (#24335)

Follow  #22719

### Major changes

1. `ServerError` doesn't do format, so remove the `%s`
2. Simplify `RenderBranchFeed` (slightly)
3. Remove unused `BranchFeedRSS`
4. Make `feed.RenderBranchFeed` respect `EnableFeed` config
5. Make `RepoBranchTagSelector.vue` respect `EnableFeed` setting,
otherwise there is always RSS icon
6. The `(branchURLPrefix + item.url).replace('src', 'rss')` doesn't seem
right for all cases, for example, the string `src` could appear in
`branchURLPrefix`, so we need a separate `rssURLPrefix`
7. The `<a>` in Vue menu needs `@click.stop`, otherwise the menu itself
would be triggered at the same time
8. Change `<a><button></button></a>` to `<a role=button>`
9. Use `{{PathEscapeSegments .TreePath}}` instead of `{{range $i, $v :=
.TreeNames}}/{{$v}}{{end}}`


Screenshot of changed parts:

<details>


![image](https://user-images.githubusercontent.com/2114189/234315538-66603694-9093-48a8-af33-83575fd7a018.png)


![image](https://user-images.githubusercontent.com/2114189/234315786-f1efa60b-012e-490b-8ce2-d448dc6fe5c9.png)


![image](https://user-images.githubusercontent.com/2114189/234334941-446941bc-1baa-4256-8850-ccc439476cda.png)

</details>


### Other thoughts

Should we remove the RSS icon from the branch dropdown list? It seems
too complex for a list UI, and users already have the chance to get the
RSS feed URL from "branches" page.

---------

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: silverwind <me@silverwind.io>

templates/repo/branch_dropdown.tmpl

@@ -42,6 +42,8 @@
 		'branches': {{.root.Branches}},
 		'tags': {{.root.Tags}},
 		'defaultBranch': {{$defaultBranch}},
+		'enableFeed': {{.root.EnableFeed}},
+		'rssURLPrefix': '{{$.root.RepoLink}}/rss/branch/',
 		'branchURLPrefix': '{{if .branchURLPrefix}}{{.branchURLPrefix}}{{else}}{{$.root.RepoLink}}/{{if $.root.PageIsCommits}}commits{{else}}src{{end}}/branch/{{end}}',
 		'branchURLSuffix': '{{if .branchURLSuffix}}{{.branchURLSuffix}}{{else}}{{if $.root.TreePath}}/{{PathEscapeSegments $.root.TreePath}}{{end}}{{end}}',
 		'tagURLPrefix': '{{if .tagURLPrefix}}{{.tagURLPrefix}}{{else if .release}}{{$.root.RepoLink}}/compare/{{else}}{{$.root.RepoLink}}/{{if $.root.PageIsCommits}}commits{{else}}src{{end}}/tag/{{end}}',