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/email-branding/select-branding.html

@@ -1,6 +1,4 @@
 {% extends "views/platform-admin/_base_template.html" %}
-{% from "components/radios.html" import radios %}
-{% from "components/page-footer.html" import page_footer %}
 {% from "components/live-search.html" import live_search %}
 
 {% block per_page_title %}

app/templates/views/letter-branding/manage-letter-branding.html

@@ -27,7 +27,7 @@
             'Save',
             button_name='operation',
             button_value='branding-details',
-            back_link=url_for('main.platform_admin'),
+            back_link=url_for('main.letter_branding'),
             back_link_text='Back to letter branding selection',
           ) }}
         </div>

app/templates/views/letter-branding/select-letter-branding.html

@@ -0,0 +1,36 @@
+{% extends "views/platform-admin/_base_template.html" %}
+{% from "components/live-search.html" import live_search %}
+
+{% block per_page_title %}
+  Letter branding
+{% endblock %}
+
+{% block platform_admin_content %}
+
+  <div class="grid-row bottom-gutter-2-3">
+    <div class="column-two-thirds">
+      <h1 class="heading-large">Letter branding</h1>
+    </div>
+    <div class="column-one-third">
+      <a href="{{ url_for('.create_letter_branding') }}" class="button align-with-heading">Add new brand</a>
+    </div>
+  </div>
+  {{ live_search(target_selector='.letter-brand', show=True, form=search_form) }}
+  <nav>
+    {% for brand in letter_brandings %}
+      <div class="letter-brand">
+        <div class="message-name">
+          {{ brand.name }}
+        </div>
+        <p class="message-type">
+          {% if brand.domain %}
+            Default for {{ brand.domain }}
+          {% else %}
+            –
+          {% endif %}
+        </p>
+      </div>
+    {% endfor %}
+  </nav>
+
+{% endblock %}

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')),