Make handling of None values in forms clearer

If the browser posts the value of `<input value='None'>` to the server
it does so as a string.

We want to post a value of `None` (actually JSON `null`) to the API. To
do this we:
- set the value in the form class to `'None'` (ie a string)
- convert to `None` (as a type) afterwards

However seeing `x = 'None'` in code looks a bit like a mistake. So to
make sure it looks deliberate and clear what is happening this commit:
- makes a reusable constant for `'None'`
- adds a comment explaining why it’s a string

app/main/views/service_settings.py

@@ -813,9 +813,8 @@ def service_preview_email_branding(service_id):
     form = ServicePreviewBranding(branding_style=branding_style)
 
     if form.validate_on_submit():
-        branding_style = None if form.branding_style.data == 'None' else form.branding_style.data
         current_service.update(
-            email_branding=branding_style
+            email_branding=form.branding_style.data
         )
         return redirect(url_for('.service_settings', service_id=service_id))