Refactor some of the logic into form

It’s messy having a lot of logic in the view methods. Handling the form
stuff can be better encapsulated inside the `Form` subclass itself.

app/main/forms.py

@@ -682,15 +682,37 @@ class ServiceSwitchLettersForm(StripWhitespaceForm):
     )
 
 
+class BrandingStyle(RadioField):
+
+    def post_validate(self, form, validation_stopped):
+        if self.data == 'None':
+            self.data = None
+
+
 class ServiceSetBranding(StripWhitespaceForm):
 
-    branding_style = RadioField(
+    branding_style = BrandingStyle(
         'Branding style',
         validators=[
             DataRequired()
         ]
     )
 
+    DEFAULT = ('None', 'GOV.UK')
+
+    def __init__(self, all_email_brandings, current_email_branding):
+
+        super().__init__(branding_style=current_email_branding)
+
+        self.branding_style.choices = sorted(
+            all_email_brandings + [self.DEFAULT],
+            key=lambda branding: (
+                branding[0] != current_email_branding,
+                branding[0] is not self.DEFAULT[0],
+                branding[1].lower(),
+            ),
+        )
+
 
 class ServicePreviewBranding(StripWhitespaceForm):
 

app/main/views/service_settings.py

@@ -887,24 +887,17 @@ def set_free_sms_allowance(service_id):
 def service_set_email_branding(service_id):
     email_branding = email_branding_client.get_all_email_branding()
 
-    form = ServiceSetBranding()
-
-    # dynamically create org choices, including the null option
-    form.branding_style.choices = sorted(
-        get_branding_as_value_and_label(email_branding) + [('None', 'GOV.UK')],
-        key=lambda branding: (
-            branding[0] != current_service.email_branding,
-            branding[0] is not 'None',
-            branding[1].lower(),
-        ),
+    form = ServiceSetBranding(
+        all_email_brandings=get_branding_as_value_and_label(email_branding),
+        current_email_branding=current_service.email_branding,
     )
 
     if form.validate_on_submit():
-        branding_style = None if form.branding_style.data == 'None' else form.branding_style.data
-        return redirect(url_for('.service_preview_email_branding', service_id=service_id,
-                        branding_style=branding_style))
-
-    form.branding_style.data = current_service['email_branding'] or 'None'
+        return redirect(url_for(
+            '.service_preview_email_branding',
+            service_id=service_id,
+            branding_style=form.branding_style.data,
+        ))
 
     return render_template(
         'views/service-settings/set-email-branding.html',