Use the proper GitLab URL for links in Wiki

Fixes gitlab-org/gitlab-ce#17071

wiki links are proper compiled, e.g.
```
[same-level](same-level) -> <a href="same-level">same-level</a>
[sub-level](sub/level) -> <a href="sub/level">sub-level</a>
[upper-level](../upper-level) -> <a href="../upper-level">upper-level</a>
```

See merge request !4026