Change naming in code to use 'postage'

Renamed everything that was previously called 'letter class' or 'postage
class' to 'postage', which is the new name we have decided to use.

app/main/forms.py

@@ -684,9 +684,9 @@ class ServiceSwitchLettersForm(StripWhitespaceForm):
     )
 
 
-class ServiceLetterClassForm(StripWhitespaceForm):
+class ServicePostageForm(StripWhitespaceForm):
 
-    letter_class = RadioField(
+    postage = RadioField(
         'Choose your postage',
         choices=[
             ('first', 'First class only'),