Backport https://github.com/go-gitea/gitea/pull/25652
This will prevent the most common cases of SVG shrinking because lack of
space. I evaluated multiple options and this seems to be the one with
the least impact in size and processing cost, so I went with it.
Unfortunately, CSS can not dynamically convert `16` obtained from
`attr()` to `16px`, or else a generic solution for all sizes would have
been possible. But a solution is [in
sight](https://developer.mozilla.org/en-US/docs/Web/CSS/attr#type-or-unit)
with `attr(width px)` but no browser supports it currently.
