fix navigation dropdown relative links

[casperdcl]

# _data/navigation.yml
- title: Test
  url: "/"
  side: left
  dropdown:
  - title: Relative
    url: "/subpage/"           # works (becomes `{{site_url}}{{baseurl}}/subpage/`)
  - title: Absolute
    url: "https://github.com"  # was broken (became `{{baseurl}}https://github.com`)