Convert radios on set-template-sender page

Includes making the legend the h1 for the page to
follow design system guidance:

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

app/main/forms.py

@@ -2021,7 +2021,7 @@ class SetTemplateSenderForm(StripWhitespaceForm):
         self.sender.choices = kwargs['sender_choices']
         self.sender.label.text = 'Select your sender'
 
-    sender = RadioField()
+    sender = GovukRadiosField()
 
 
 class LinkOrganisationsForm(StripWhitespaceForm):