mode-based feature flags

This PR makes it easier to have features with a modality-related on/off flag - for example UIKonf doesn't use the 'Motivations' feature. It adds mode-related helpers that wrap the common query cases -- that someone is signed in and can X Y, or that someone is not signed in and... -- in the view helpers, and that can be mixed into Controllers, Models or anywhere else you think you need them.

It also introduces an AnonymousUser class, which represents an anonymous user, allowing us to eliminate nil-as-anonymous-user and some conditional logic.

This closes #38