Provide feedback on username_available call (Closes #134)

From the issue:

We should get the spinner in there beside the "Next" button as we do in the newsletter step.

That's what this commit does. The extra noise is because I didn't like how hasRegisterStarted sounded when doing the username_available check; I wanted something more generic sounding. So I made hasSubmitStarted and hasSubmitFinished instead. I'm about to open an issue where I want to discuss if we should even be doing this at all anymore now that ngrx-forms is a thing, but in the meantime I wanted to have a fix for this issue right now.

From here down in the reducer file is where most of the action happens.