Improve Tor best practices tracker

Various improvements could be done to the best-practices tracker (practracker #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.

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