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

[Quentin Gliech]

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