Allow sublocation hosting

I'd like to selfhost an instance of this but in a sublocation of an existing domain, e.g. `https://gitlab.example.org/shields/`.

As far as I can tell, the only required change would be to make the `pathname` attribute in the following code configurable:

https://github.com/badges/shields/blob/eb969fd442c5a56f2fb0cf0b5b679982be02ec97/core/server/server.js#L300-L312

for example:

```js
pathname: this.config.public.base_url,
```

Start in core/server/server.js at the referenced lines and inspect how the pathname is currently set, along with the public.base_url configuration. Verify the behavior with a self-hosted instance under a URL prefix and confirm that generated links and routing work from that sublocation.

Written by the indexing model from the issue text.