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
This moves `hostName` to the `Collection` struct, where it's needed. The
field is populated after successful `GetCollection...()` calls.
This isn't the cleanest way to do things, but it accomplishes the goal.
Eventually, we should accept the AppCfg to `GetCollection...()` calls,
or make them `App` methods, instead of `datastore` methods.
Ref T613
This helps with post importing and clients that want to support post
scheduling. It also changes how Collection.ForPublic() works, no longer
resetting the ID.
Closes T532