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-line-arrow-down.svg

@@ -0,0 +1 @@
+<svg viewBox="0 0 16 16" class="svg octicon-line-arrow-down" width="16" height="16" aria-hidden="true"><path d="M4.5 7.5L3 9l5 5 5-5-1.5-1.5L9 10.179V2H7v8.179L4.5 7.5z"/></svg>