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.

web_src/js/components/RepoBranchTagSelector.vue

@@ -39,6 +39,9 @@
       <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')">
+            <svg-icon name="octicon-rss" :size="14"/>
+          </a>
         </div>
         <div class="item" v-if="showCreateNewBranch" :class="{active: active === filteredItems.length}" :ref="'listItem' + filteredItems.length">
           <a href="#" @click="createNewBranch()">