Previously, we had a `showFooter` boolean on the application controller which would be set true/false in various routes by different routes/controllers. A global `routeWillChange` hook would set it `false` before every route transition, and the destination route/controller would have to set it `true` for the footer to show correctly.
This commit replaces that with a new 'declarative' system. Instead of having to set the value true/false manually, UIs which need the footer to be hidden can simply include the `{{hide-application-footer}}` helper in their template when needed. The helper/service will automatically keep track of all the current invocations of that helper, and only show the footer when there are 0 invocations.
This significantly simplifies things, and removes the need for many observers and controller injections, both of which are considered 'code smells' in modern Ember applications.
