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

* Move pull request api convert to convert package

* Rename ToPullRequest to ToAPIPullRequest

* Fix counts on issues dashboard

* setupSess -> setupSession

* Unit test

* Load repo owners for issues