Convert radios on set-sender page

Includes making the legend the page header to
follow design system guidance:

https://design-system.service.gov.uk/get-started/labels-legends-headings/

I had to add some override styles to allow the
legend to classes as a full-width column, and the
radios to be in one that is three-quarters.

app/main/forms.py

@@ -2011,7 +2011,7 @@ class SetSenderForm(StripWhitespaceForm):
         self.sender.choices = kwargs['sender_choices']
         self.sender.label.text = kwargs['sender_label']
 
-    sender = RadioField()
+    sender = GovukRadiosField()
 
 
 class SetTemplateSenderForm(StripWhitespaceForm):