Point users to Power Pack and support contracts after a site install.

After a user installs a site with 'rb-site install', they're now given
links to Power Pack and to our support contracts page. These are
benefits that we historically haven't done a great job pointing new
users to, so we want them to at least know about them as they're getting
going.

We even make the Power Pack trial process a bit easier by giving users
their install key up-front.

Reviewed at https://reviews.reviewboard.org/r/6020/