404.html.haml

This enables us to exploit the advantages of middleman URL management.
For example, this will allow us to host review apps in subdomains (see
https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/8203 for
more information).

The regular expression `%a((?:\.[\w-]+)+)\{ ?href:
?"([\w\/\-_]+)" ?\} (.+)` was used substituting by `= link_to "$3",
"$2", class: "$1"`. Other cases were derived from this. Then class names
were adjusted by replacing `class: "\.([\w\.-]+)"` with `class: "$1"`
once and `class: "([\w -]+)\.([\w\.-]+)"` with `class: "$1 $2"` as many
times as necessary. There might be a better approach but this is what I
came up with :)