When running rb-site manage ... <command> --help, we'd show the

standard management command help text that Django displays. This would

end up showing the wrong program invocation, which wasn't helpful. For

instance:
rbsite.py set-siteconfig

When we wanted something like:
rb-site manage /var/www/reviewboard set-siteconfig

This change fixes that by ditching the execute_from_command_line()

wrapper function and instead creating a ManagementUtility directly,

where we set the correct program name in order to get the right command

in the standard manage.py --help list of commands. We then pass in

rb-site-manage as a placeholder program name, which gets replaced in a

monkey-patched parser wrapper.
This is, unfortunately, the only way to do that without outright

replacing Django management command infrastructure, but should be safe

to do.

Tested both rb-site manage /path/to/site -- --help and

rb-site manage /path/to/site <command> -- --help for several different

commands. Saw a useful "usage:" line in the help with the full command.