Reorganized planet page to put contributor list at bottom

As it might grow quickly, it's better to make it not obscure the
information on how to become a contributor, which is probably what will
be more useful now in the start.
The GitHub ribbon is now implemented in CSS

This allowed us to change the text to something slightly less offensive
(for those with tender hearts).

Some old browsers don't support this well, if it bothers us to much we
could try to include a polyfill of some kind.
Increase slightly the visibility of the page subtitle