`http.public_base` is not behaving as expected without a trailing slash, leading to confusing errors

*This issue was originally created by [**@sandhose**](https://github.com/sandhose) at .*

If one sets the `public_base` to something like `https://domain/account`, MAS will still construct URLs with `https://domain/` as the base.

This is because of the behaviour of `Url::join`, which is sensitive to trailing slashes on the base URL. We should either add the trailing slash (and issue a warning) if it's missing, or even refuse to start