Fix wiki links when `base-url` is set to empty and `absolute-urls` to yes

In config file, when `base-url` is set to empty (or `/`, which will then be trailed to empty) and `absolute-urls` set to yes, the wiki links doesn't actually use absolute urls, since it will use `</>` to concat `base-url` and the wiki link.

So in page `/Sub/Page`, the link `[OtherPage]` will produce the code `<a href="OtherPage">` thus point to `/Sub/OtherPage`, while it should produce the code `<a href="/OtherPage">`.

We should always concat a `'/'` before the `base-url` to make sure the produced url is an absolute url.