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

We load it always, no need for a user config UI

This is a first stab at having a configurable code highlighting option,
similar to the MathJax rendering option. This change makes a checkbox
in the settings for code highlighting using the highlightjs.org
library.
What works: code highlighting in multi-user env is like I would
expect. single and anon(?) needs work

Things to resolve/consider:
- does the .IsCode test for code highlighting need to stay? At least
this and that should use the same version of the highlight.js lib.
- can the common templating part be 'included' somehow?
- the anon vs single-user vs multi-user code is not completely
clear (to me)
- bring js to local instead of cloudfare cdn (perhaps combine with
MathJax)

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

This includes config changes, collections, posts, some post rendering
funcs, and actual database connection when the server starts up.