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](365bb77a54/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>
a96c73f979