baseurl of '/' now breaks on GitHub pages

### Before submitting an issue, please be sure to

- [x] Read [the contributing instructions](https://github.com/github/pages-gem/blob/master/CONTRIBUTING.md#contributing-to-the-github-pages-gem)
- [x] Update to the latest Gem version (run `bundle update github-pages`)

### This issue affects

- [x] The site generated by GitHub Pages
- [ ] Building sites locally

### What did you do (e.g., steps to reproduce)

I pushed my site with a small change from the last push couple of weeks ago.

### What did you expect to happen?

The site to appear as it has done for years :)

### What happened instead?

All the styling is lost, because the upgrade to jekyll-github-metadata v2.5.0 means that a config with baseurl set to `/` is now overridden to baseurl `""` and the templates were relying on baseurl being set to `/`. I understand that `baseurl: "/"` is incorrect and shouldn't be used (though I can't find any official documentation saying why this shouldn't work okay, only links to Parker's [2014 blog post](https://byparker.com/blog/2014/clearing-up-confusion-around-baseurl/)), but anyway, I didn't expect the whole site to break by adding a new blog post :)

### Additional information

* Link to the live site (if applicable): http://fixmystreet.org/overview/ (now fixed by https://github.com/mysociety/fixmystreet/commit/f3bb932c1b7f09fd4bffef2ef403c11008f9fcb0)
* Link to the source repo (if applicable): https://github.com/mysociety/fixmystreet/tree/a1dfd4acb844162227c27d4e19c6e863cc96a692

Beginne damit, den GitHub-Pages-Build mit auf "/" gesetztem baseurl und jekyll-github-metadata v2.5.0 nachzubilden, wobei das verlinkte Quell-Repository und die Live-Site als Referenzen dienen. Verfolge, wie der Konfigurationswert zu den Templates gelangt, und identifiziere die relevanten generierten Asset-URLs; als erledigt gilt dies, wenn die Site ihr Styling beibehält, ohne das dokumentierte baseurl-Verhalten zu beeinträchtigen.

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