Improve <SvgIcon> to make it output svg node and optimize performance (#23570)

Before, the Vue `<SvgIcon>` always outputs DOM nodes like: 

```html
<span class="outer-class">
    <svg class="class-name-defined" ...></svg>
</span>
```

The `span` is redundant and I guess such layout and the inconsistent
`class/class-name` attributes would cause bugs sooner or later.

This PR makes the `<SvgIcon>` clear, and it's faster than before,
because it doesn't need to parse the whole SVG string.

Before: 

<details>

![image](https://user-images.githubusercontent.com/2114189/226156474-ce2c57cd-b869-486a-b75b-1eebdac8cdf7.png)

</details>

After:

![image](https://user-images.githubusercontent.com/2114189/226155774-108f49ed-7512-40c3-94a2-a6e8da18063d.png)

---------

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

web_src/js/components/ContextPopup.vue

@@ -3,7 +3,7 @@
     <div v-if="loading" class="ui active centered inline loader"/>
     <div v-if="!loading && issue !== null">
       <p><small>{{ issue.repository.full_name }} on {{ createdAt }}</small></p>
-      <p><svg-icon :name="icon" :class="[color]" /> <strong>{{ issue.title }}</strong> #{{ issue.number }}</p>
+      <p><svg-icon :name="icon" :class="['text', color]" /> <strong>{{ issue.title }}</strong> #{{ issue.number }}</p>
       <p>{{ body }}</p>
       <div>
         <div