Don’t show postage choice for international letters

International letters don’t have a choice of postage. Under the hood
they are either `europe` or `rest-of-world`.

So, for letters that we detect are international, this commit:
- removes the radios buttons that give users the choice of postage
- passes through either `europe` or `rest-of-world` to the API,
  depending on what address we find in the letter

This will cause the API to 500 until it can accept `europe` or
`rest-of-world` as postage types, but this is probably OK because it’s
only our services that have international letters switched on at the
moment.

app/main/forms.py

@@ -9,6 +9,7 @@ from flask_wtf import FlaskForm as Form
 from flask_wtf.file import FileAllowed
 from flask_wtf.file import FileField as FileField_wtf
 from notifications_utils.columns import Columns
+from notifications_utils.countries.data import Postage
 from notifications_utils.formatters import strip_whitespace
 from notifications_utils.postal_address import PostalAddress
 from notifications_utils.recipients import (
@@ -850,6 +851,19 @@ class LetterTemplatePostageForm(StripWhitespaceForm):
 
 
 class LetterUploadPostageForm(StripWhitespaceForm):
+
+    def __init__(self, *args, postage_zone, **kwargs):
+
+        super().__init__(*args, **kwargs)
+
+        if postage_zone != Postage.UK:
+            self.postage.choices = [(postage_zone, '')]
+            self.postage.data = postage_zone
+
+    @property
+    def show_postage(self):
+        return len(self.postage.choices) > 1
+
     postage = RadioField(
         'Choose the postage for this letter',
         choices=[