Let users switch channels on and off by themselves

We let people do this for letters already. We should let them do it for
emails and texts too, rather than have to email us.

app/main/forms.py

@@ -696,10 +696,18 @@ class ServiceLetterContactBlockForm(StripWhitespaceForm):
             )
 
 
-class ServiceSwitchLettersForm(StripWhitespaceForm):
+class ServiceSwitchChannelForm(StripWhitespaceForm):
+
+    def __init__(self, channel, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        self.enabled.label.text = 'Send {}'.format({
+            'email': 'emails',
+            'sms': 'text messages',
+            'letter': 'letters',
+        }.get(channel))
 
     enabled = RadioField(
-        'Send letters',
+        'Choices',
         choices=[
             ('on', 'On'),
             ('off', 'Off'),