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-file-symlink-directory.svg

@@ -0,0 +1 @@
+<svg viewBox="0 0 14 16" class="svg octicon-file-symlink-directory" width="16" height="16" aria-hidden="true"><path fill-rule="evenodd" d="M13 4H7V3c0-.66-.31-1-1-1H1c-.55 0-1 .45-1 1v10c0 .55.45 1 1 1h12c.55 0 1-.45 1-1V5c0-.55-.45-1-1-1zM1 3h5v1H1V3zm6 9v-2c-.98-.02-1.84.22-2.55.7-.71.48-1.19 1.25-1.45 2.3.02-1.64.39-2.88 1.13-3.73C4.86 8.43 5.82 8 7.01 8V6l4 3-4 3H7z"/></svg>