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
Matt Baer
Noëlle Anthony