Tell people they can have their logo on letters

At the moment there’s nothing in Notify that says the logo on letter
templates can be changed from ‘HM Government’. While some people guess
that it’s possible, or are motivated enough to enquire, others might be
assuming that the branding can’t be changed (and thus deciding Notify
letters aren’t for them).

In the future we should make this process slicker (eg with a file
upload) but the easiest, quickest improvement to make for now is:
- let people know that the branding can be changed
- direct them to support if they do want to change it

app/navigation.py

@@ -189,6 +189,7 @@ class HeaderNavigation(Navigation):
         'registration_continue',
         'remove_user_from_organisation',
         'remove_user_from_service',
+        'request_letter_branding',
         'request_to_go_live',
         'resend_email_link',
         'resend_email_verification',
@@ -325,6 +326,7 @@ class MainNavigation(Navigation):
         'settings': {
             'branding_request',
             'link_service_to_organisation',
+            'request_letter_branding',
             'request_to_go_live',
             'service_add_email_reply_to',
             'service_add_letter_contact',
@@ -632,6 +634,7 @@ class CaseworkNavigation(Navigation):
         'registration_continue',
         'remove_user_from_organisation',
         'remove_user_from_service',
+        'request_letter_branding',
         'request_to_go_live',
         'resend_email_link',
         'resend_email_verification',
@@ -853,6 +856,7 @@ class OrgNavigation(Navigation):
         'register_from_org_invite',
         'registration_continue',
         'remove_user_from_service',
+        'request_letter_branding',
         'request_to_go_live',
         'resend_email_link',
         'resend_email_verification',