Add site-wide navigation on colls when chorus = true

This adds a new config value: chorus that signifies an instance is
more about the Reader view than individual blogs / writers. When
enabled, user navigation will show on all pages, including About,
Reader, and Privacy (ref T680).

It also uses different collection templates that keep the instance-wide
navigation at the top of the page, instead of the author's name --
again, branded more for the collective than the individual.

Ref T681