Ember app improvements

Some slightly stylistic changes more recent versions of Ember let's us do.

A combination of children route getting their parent model by default and being able to supply a route's template name let's us use {{outlet}} instead of mutating the state on a shared controller object.

Automatic component lookup let's us remove the explicit registration of the todo input. We can also re-use the component for both new todos and editing todos.

Closes #943