Add page where users can say they want branding

At the moment branding is an undocumented feature. We get a bunch of
support tickets from teams asking its possible.

This commit:
- lets people know it’s possible, and what the options are
- is the first step towards making this process as self-service as
  possible

In some cases we will be able to infer a user’s organisation from there
email address, and Google image search their logo. So the experience for
them is that they press a button and government just sorts it out for
you (also known as "the dream").

In other cases we will have to get back to people asking for a copy of
their logo, or to find out about their service, but this is what we have
to do at the moment anyway.

app/templates/views/service-settings.html

@@ -89,6 +89,17 @@
             }}
           {% endcall %}
 
+          {% call row() %}
+            {{ text_field('Email branding') }}
+            {{ text_field(
+              'GOV.UK' if current_service.branding == 'govuk' else 'Your branding'
+            ) }}
+            {{ edit_field(
+              'Change',
+              url_for('.branding_request', service_id=current_service.id),
+            )}}
+          {% endcall %}
+
         {% endif %}
 
       {% endcall %}