Remove svg.svg class, restore .rss-icon (#24667)

Fix regression from https://github.com/go-gitea/gitea/pull/24476 where
the `svg.svg` class misaligns SVG icons across the site and streched
buttons unintentionally in vertical height.

Before (button 30.3px):
<img width="157" alt="Screenshot 2023-05-11 at 22 09 42"
src="https://github.com/go-gitea/gitea/assets/115237/0fd137ab-ab52-4cf8-afca-c45776d526d0">

After (button 30px):
<img width="160" alt="Screenshot 2023-05-11 at 22 09 59"
src="https://github.com/go-gitea/gitea/assets/115237/4b741f4b-0fd2-4fae-9bee-16a7deb098e8">

[vertical-align:
middle](https://developer.mozilla.org/en-US/docs/Web/CSS/vertical-align)
is not suitable to align icons to text because

> Aligns the middle of the element with the baseline plus half the
x-height of the parent.

Example of `vertical-align: middle` from MDN:

<img width="232" alt="Screenshot 2023-05-11 at 22 29 28"
src="https://github.com/go-gitea/gitea/assets/115237/179fb756-85a1-4cab-8219-1a4958f333e2">

So I think the
[existing](https://github.com/go-gitea/gitea/blob/365bb77a5497d492836bf823cf780c823db27e8c/web_src/css/svg.css#L3)
`vertical-align: text-top` is generally still the best bet:

<img width="241" alt="Screenshot 2023-05-11 at 22 34 24"
src="https://github.com/go-gitea/gitea/assets/115237/0cd6edf5-12c0-4bdb-8771-a900f5ba2d35">

Co-authored-by: Giteabot <teabot@gitea.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="enableFeed && mode === 'branches'" role="button" class="ui compact right muted" :href="rssURLPrefix + item.url" target="_blank" @click.stop>
+          <a v-if="enableFeed && mode === 'branches'" role="button" class="rss-icon ui compact right" :href="rssURLPrefix + item.url" target="_blank" @click.stop>
             <svg-icon name="octicon-rss" :size="14"/>
           </a>
         </div>