Direct SVG rendering (#12157)

Introduce 'make svg' which calls a node script that compiles svg files
to `public/img/svg`. These files are vendored to not create a dependency
on Node for the backend build.

On the frontend side, configure webpack using `raw-loader` so SVGs can
be imported as string.

Also moved our existing SVGs to web_src/svg for consistency.

Fixes: https://github.com/go-gitea/gitea/issues/11618

public/img/svg/octicon-chevron-down.svg

@@ -0,0 +1 @@
+<svg viewBox="0 0 10 16" class="svg octicon-chevron-down" width="16" height="16" aria-hidden="true"><path fill-rule="evenodd" d="M5 11L0 6l1.5-1.5L5 8.25 8.5 4.5 10 6l-5 5z"/></svg>