don't add an empty country.
don't bother the user with mcc/mnc in any way.

Previously, if no suitable sim card was inserted, the user had to select 1. a carrier and 2. a mcc/mnc combination.
Now we simply let the user choose a carrier and take the first possible mcc/mnc combination for this carrier.
Fix error where 'Carrier' was not shown in the carrier selection box before choosing any carrier.
delete leftovers from mcc/mnc selections; make disabled submit button on init properly
minor bugfixes; make disabled prefix-selector on confirmation page more clear
fix minor bugs