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/templates/views/platform-admin/_base_template.html

@@ -18,6 +18,7 @@
           ('Organisations', url_for('main.organisations')),
           ('Providers', url_for('main.view_providers')),
           ('Email branding', url_for('main.email_branding')),
+          ('Letter branding', url_for('main.letter_branding')),
           ('Inbound SMS numbers', url_for('main.inbound_sms_admin')),
           ('Find users by email', url_for('main.find_users_by_email')),
           ('Email Complaints', url_for('main.platform_admin_list_complaints')),