Bug 961184 - Part 1: Add FxAccountClient*.resendCode.
Bug 961184 - Part 2: Rework activities shown after CreateAccount and SignIn.
Bug 961184 - Part 3: Implement sending verification code from ConfirmAccount activity.
Bug 961184 - Part 4: Disable link when verification code cannot be sent.
Bug 961184 - Show activity after signing in to account. r=rnewman

Also (partially) implements resending verification codes, but doesn't
yet implement resending after account creation.