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>

web_src/js/components/RepoBranchTagSelector.vue

@@ -39,7 +39,7 @@
       <div class="scrolling menu" ref="scrollContainer">
         <div v-for="(item, index) in filteredItems" :key="item.name" class="item" :class="{selected: item.selected, active: active === index}" @click="selectItem(item)" :ref="'listItem' + index">
           {{ item.name }}
-          <a v-if="mode === 'branches'" role="button" class="ui compact muted right" :href="(branchURLPrefix + item.url).replace('src', 'rss')">
+          <a v-if="enableFeed && mode === 'branches'" role="button" class="ui compact muted right" :href="rssURLPrefix + item.url" target="_blank" @click.stop>
             <svg-icon name="octicon-rss" :size="14"/>
           </a>
         </div>
@@ -294,3 +294,12 @@ export function initRepoBranchTagSelector(selector) {
 
 export default sfc; // activate IDE's Vue plugin
 </script>
+
+<style scoped>
+.menu .item a {
+  display: none; /* only show RSS icon on hover */
+}
+.menu .item:hover a {
+  display: inline-block;
+}
+</style>