Allow updates to SMS prefixing setting

We’re extracting this from being determined based on what the sender
name is to its own setting.

This commit will let users set it independently.

Until the explicitly set it, it will still be determined based on
whether their default sender name matches the default for the platform.

app/main/forms.py

@@ -721,6 +721,16 @@ class InternationalSMSForm(Form):
     )
 
 
+class SMSPrefixForm(Form):
+    enabled = RadioField(
+        'Start all text messages with service name',
+        choices=[
+            ('on', 'On'),
+            ('off', 'Off'),
+        ],
+    )
+
+
 def get_placeholder_form_instance(
     placeholder_name,
     dict_to_populate_from,