Allow services to turn international SMS on/off

We didn’t make this self-service before because the pricing information
wasn’t published (ie we had to send it to services that asked for it).

Now that we publish pricing information in the app, there’s no reason
why services can’t make an informed decision about whether they want
international SMS or not.

So this commit:
- removes the platform admin button
- adds some radio buttons that our users can click with their mice

app/main/forms.py

@@ -658,6 +658,16 @@ class ServiceInboundApiForm(Form):
                                                            Length(min=10, message='Must be at least 10 characters')])
 
 
+class InternationalSMSForm(Form):
+    enabled = RadioField(
+        'Send text messages to international phone numbers',
+        choices=[
+            ('on', 'On'),
+            ('off', 'Off'),
+        ],
+    )
+
+
 def get_placeholder_form_instance(
     placeholder_name,
     dict_to_populate_from,