correct edge case & apply suggestions; enhance :)

The edge case is that until now, we simply ask the user for mcc/mnc if the selected country does not have the mcc/mnc of the sim. It's probably better to check first whether we have the mcc/mnc combination at all - if we don't; assume the user still wants to use the sim card inserted.
Not knowing the mcc/mnc combination still may not yield best results (e.g. displayed sim cards), but dual-sim + unknown mcc/mnc combination can probably be seen as very unlikely

The following suggestions from @willyaranda have been applied:

* Remove "Welcome to OpenWapp"
* Combine welcome-message 1 & 2
* Remove T&C
* Don't bother users with mcc & mnc at all, if they select a carrier, simply take the first mcc/mnc combination that matches

Additionally, the following enhancements have been made:

* put all found mcc/mnc combinations in a map, so the associated country can be found easily
* don't add an empty country.
* 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
* don't use array.from due to support for older versions:
  Navigator.mozMobileConnections started to exist with ffos 1.3 [1], which is based on gecko 28 [2], while array.from started to exist with gecko 32 [3]

[1] https://developer.mozilla.org/en-US/docs/Web/API/Navigator.mozMobileConnections
[2] https://developer.mozilla.org/en-US/Firefox_OS/Releases/1.3
[3] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/from#Browser_compatibility