Include some help text on the FxA email form (bug 1095626)

Changes in https://github.com/mozilla/marketplace-core-modules/pull/9 too.
I am [calling `showEmailForm` explicitly](https://github.com/mozilla/fireplace/pull/827/files#diff-bc28e6e85c8b7bbc752e2adb60001847R101) now so it will get set there.

Previously if the server took a while to return the account status you'd briefly see the email form then be redirected to FxA if you were signed in with Persona. With this change you shouldn't see anything.