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>

tests/integration/pull_create_test.go

@@ -22,7 +22,7 @@ func testPullCreate(t *testing.T, session *TestSession, user, repo, branch, titl
 
 	// Click the PR button to create a pull
 	htmlDoc := NewHTMLParser(t, resp.Body)
-	link, exists := htmlDoc.doc.Find("#new-pull-request").Parent().Attr("href")
+	link, exists := htmlDoc.doc.Find("#new-pull-request").Attr("href")
 	assert.True(t, exists, "The template has changed")
 	if branch != "master" {
 		link = strings.Replace(link, ":master", ":"+branch, 1)