Modify register step form for guests

• Passport Country now displays two letter code after selection.
• A heading has been added for identity information.
• An error message is now displayed if the user inputs both national id and passport information.

Resolves: GREG-120

frontend/public/locales/en/common.json

@@ -100,6 +100,7 @@
     "startDateMustBeSet": "Start date must be set",
     "startDateMustBeBeforeEndDate": "Start date has to be before end date",
     "passportNationalityAndNumber": "Both passport nationality and number need to be set",
+    "doubleIdentity": "*You have input both national ID and passport. Please choose only one.",
     "nationalIdOrPassport": "National ID or passport information need to be entered"
   },
   "button": {
@@ -134,6 +135,8 @@
   "registerNewGuest": "Register new guest",
   "guestOverview": "Guest overview",
   "guestRegisterWizardText": {
+    "identityHeader": "Identify yourself ",
+    "identityBody": "Enter national identity number if you have one. <1>Otherwise</1> use passport information.",
     "yourContactInformation": "Your contact information",
     "contactInformationDescription": "Fill in your mobile phone number.",
     "yourGuestPeriod": "Your guest period",