MB-100: really hide hidden submit on 1st step of wizard

Initially this button was visibility:hidden, but we found it prevents
Enter from submitting form on IE. See
b01dd0ec933eb69257fd635d37609d6521463da1.

But clearly just forcing it to have 0 width and height doesn't
completely hide it due to possibly non-zero border, padding and
margin. But fixing that is easy too.

Change-Id: I60c9de04efbaac5b20e802d8fe3a670ed45c5b43
Reviewed-on: http://review.couchbase.org/26610
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>