Serve nice 404 page

![Screen_Shot_2016-02-26_at_15.34.10](/uploads/bfedef869fb6f51dec0f3c730069e42e/Screen_Shot_2016-02-26_at_15.34.10.png)

See merge request !4

Present user-provided 404 pages

If the 404.html is present in root of project it will be presented instead of generic 404.

**Example:**
1. If you use the project page (served under /project/) and try to access: `/project/non/exsiting/file` we will try to serve: `/project/404.html`, then `/404.html`.

1. If you use the group page (served under /) and try to access `/non/existing/file` we will try to serve: `/404.html`.

1. If you use custom domain and try to access: `/non/existing/file` we will try to server `/404.html` only.

cc @axil 

Fixes https://gitlab.com/gitlab-org/gitlab-ee/issues/318


See merge request !2