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/navigation.py

@@ -109,6 +109,7 @@ class HeaderNavigation(Navigation):
         'api_integration',
         'api_keys',
         'archive_service',
+        'branding_request',
         'callbacks',
         'cancel_invited_org_user',
         'cancel_invited_user',
@@ -309,6 +310,7 @@ class MainNavigation(Navigation):
             'usage',
         },
         'settings': {
+            'branding_request',
             'link_service_to_organisation',
             'request_to_go_live',
             'service_add_email_reply_to',
@@ -519,6 +521,7 @@ class OrgNavigation(Navigation):
         'api_keys',
         'archive_service',
         'bat_phone',
+        'branding_request',
         'callbacks',
         'cancel_invited_org_user',
         'cancel_invited_user',