Improve Tor best practices tracker

Various improvements could be done to the best-practices tracker (practracker legacy/trac#29221 (moved)). Here are a few from Nick's last review:

https://github.com/torproject/tor/pull/742#discussion_r262564420 https://github.com/torproject/tor/pull/742#discussion_r262565153 https://github.com/torproject/tor/pull/742#discussion_r262566041 https://github.com/torproject/tor/pull/742#discussion_r262566501 https://github.com/torproject/tor/pull/742#discussion_r262567882

This could also be a fine starting volunteer project.