Inform users of missing dependencies in rb-site install.

rb-site previously didn't check for optional or required dependencies,
meaning that a user could get far into the install and then fail at
installation time due to the memcached or database modules not being
available.

This change adds a page after the Introduction page that will list the
missing dependencies, giving users a chance to bail and install them. If
they're missing support for databases altogether, then we force them to
quit.

Went through both console and GTK+ installs and tested that missing dependencies were listed and that the corresponding entries weren't enabled in the UIs.

Simulated not having any required DB modules and verified that I wasn't allowed to continue with the install process.