Make app state dependencies more explicit

[linclark]

@jsantell I'm going to break up the reducer refactor into pieces to make it easier to review. This should also help us avoid stepping on each other's toes too much as we both work on the reducers.

This is the first piece.

[coveralls]

Coverage decreased (-0.02%) to 39.969% when pulling 4edc203 on linclark:app-state-deps into 935c0d0 on mozilla:master.

[jsantell]

LGTM! Should these helpers be in reducers, though, if not used as a redux reducer? Even like ./reducers/helpers.js maybe? I don't feel strongly though

[linclark]

Good idea. We will probably have a lot more, so we probably don't want to put it in the same file. Looking back at the Redux docs for selectors, it looks like that uses a selectors/ dir.

I'll do that as part of the follow ups to this one.