Inform users of missing dependencies in rb-site install.
Review Request #820 — Created April 17, 2009 and submitted
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.