feature(client): Disable the submit button while auth server requests are outstanding.

* Decorate all calls to the fxa client with a function that messages out whenever auth server calls start and complete.
* In Form based views, listen for these messages and disable the submit button.

We could take a much simpler approach and debounce the submit handler in the form. We could also take care of disabling the submit button from fxa-client.js, but that feels dirty.

@zaach and @nchapman - thoughts?

fixes #751