Revert "Railsclub promocode"

This reverts commit 1ffca5d36f8f1824fb1a908d622449135fab462a.
Revert "Revert "Railsclub promocode""

This reverts commit b1d351e95d12b5ad9e5bb8741583f789b353d430.
Revert "Add railsclub banner"

This reverts commit 2c0dad3aa57c1c9831a68fd1f4ddfb9c87bb7374.

Conflicts:
	source/_sidebar.html.haml
	source/images/banners/railsclub_sidebar.png
S06E17