Versions dropdown menu

We need a dropdown menu that will have the available versions.

1. By default, have edge selectable which will be what we show under the version-less URL (pulled from master)
2. Have all available versions that will be possible be picked up by data/versions.yml (need to first implement https://gitlab.com/gitlab-com/gitlab-docs/issues/184)
3. The last item of the dropdown should be a link pointing to /archives where we will have info about the current available online versions and the ones available offline (via Docker images)