set baseurl to match exposed artifact preview

This makes links like stylesheets etc match where they get served up for exposed artifacts, rendering things a bit more correctly.

In addition, it enables ugly URLs so links work when previewing, because the GitLab artifact preview doesn't support something like DirectoryIndex. It's not ideal, but seems better than doing nothing...

This is effectively a reopen of !34 (closed), which somehow got broken by the master -> main rename. After thinking about this a bit more, I think this is worth it after all, because it's annoying to not be able to preview changes.