Branded login page for CE

Report https://news.ycombinator.com/item?id=10931347

Merge http://doc.gitlab.com/ee/customization/branded_login_page.html from EE into CE.

I think it makes sense for smaller organizations to need this.

When you are in the app it is branded by the GitLab icon and logo already (that also serves as loading indicator).

On the login page you have 'About GitLab' in the footer anyway.

@dzaporozhets are you OK with this? /cc @JobV @ashleys