Add '/letter-branding' page to show all letter brands

This is a platform admin page which lists all letter brands and their
domain. Once the page to edit a letter brand has been created, each
brand will link to its edit page, but for now this is just a list of
brand names and domains.

app/navigation.py

@@ -78,6 +78,7 @@ class HeaderNavigation(Navigation):
             'create_letter_branding',
             'email_branding',
             'find_users_by_email',
+            'letter_branding',
             'live_services',
             'organisations',
             'platform_admin',
@@ -437,6 +438,7 @@ class MainNavigation(Navigation):
         'information_security',
         'integration_testing',
         'invite_org_user',
+        'letter_branding',
         'live_services',
         'manage_org_users',
         'new_password',
@@ -623,6 +625,7 @@ class CaseworkNavigation(Navigation):
         'integration_testing',
         'invite_org_user',
         'invite_user',
+        'letter_branding',
         'link_service_to_organisation',
         'live_services',
         'manage_org_users',
@@ -855,6 +858,7 @@ class OrgNavigation(Navigation):
         'information_security',
         'integration_testing',
         'invite_user',
+        'letter_branding',
         'link_service_to_organisation',
         'live_services',
         'manage_template_folder',