Use same layout as other organisation pages

So it feels consistent from moving from one page to another, rather
than jumping in and out of the platform admin view.

app/navigation.py

@@ -80,7 +80,6 @@ class HeaderNavigation(Navigation):
             'user_profile_disable_platform_admin_view',
         },
         'platform-admin': {
-            'add_organisation',
             'archive_user',
             'clear_cache',
             'create_email_branding',
@@ -120,6 +119,7 @@ class HeaderNavigation(Navigation):
         'accept_org_invite',
         'action_blocked',
         'add_data_retention',
+        'add_organisation',
         'add_service',
         'add_service_template',
         'agreement',

app/templates/views/organisations/add-organisation.html

@@ -1,4 +1,4 @@
-{% extends "views/platform-admin/_base_template.html" %}
+{% extends "withoutnav_template.html" %}
 {% from "components/page-header.html" import page_header %}
 {% from "components/page-footer.html" import page_footer %}
 {% from "components/textbox.html" import textbox %}
@@ -9,18 +9,31 @@
   New organisation
 {% endblock %}
 
-{% block platform_admin_content %}
+{% block fullwidth_content %}
+  <div id="content">
+    <div class="navigation-service">
+      <div class="navigation-service-name">
+        <a href="{{ url_for('.organisations') }}">All organisations</a>
+      </div>
+      <a href="{{ url_for('main.choose_account') }}" class="navigation-service-switch">Switch service</a>
+    </div>
 
-  {{ page_header(
-    'New organisation',
-    back_link=url_for('.organisations')
-  ) }}
-
-  {% call form_wrapper() %}
-    {{textbox(form.name)}}
-    {{radios(form.organisation_type)}}
-    {{radios(form.crown_status)}}
-    {{ page_footer('Save') }}
-  {% endcall %}
+    <main role="main">
+      <div class="grid-row">
+        <div class="column-one-quarter">
+          &nbsp;
+        </div>
+        <div class="column-three-quarters">
+          {{ page_header('New organisation') }}
+          {% call form_wrapper() %}
+            {{textbox(form.name)}}
+            {{radios(form.organisation_type)}}
+            {{radios(form.crown_status)}}
+            {{ page_footer('Save') }}
+          {% endcall %}
+        </div>
+      </div>
+    </main>
+  </div>
 
 {% endblock %}