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
