Streamline rb-site install.

Many of the options rb-site install presented were advanced options that
a new admin likely didn't need to deal with. For example, the static and
media URLs, web server, Python loader, and the option for file-based
caching.

In order to simplify the first-time install experience, these wizard
pages have been hidden behind an --advanced flag, leaving only the
options that most users will need to deal with.

We also don't want to support mod_python anymore, so this has been
removed entirely.

In total, this takes us from 13 wizard pages to 8.
Allow continuing when hitting a DB sync error in rb-site.

A problem people sometimes hit is forgetting to create the Review Board
database or setting the right permissions before they proceed with
rb-site install. This can lead to a failure when synchronizing the
database, and then the need to delete the site directory and start over.

We now present a helpful error with the ability to continue and try
again as many times as needed.

Fixdes bug #2126