Go back to template if coming from template

Most users don’t have multiple contact blocks. So by default it should
feel like you’re just editing the one contact block, rather than
managing a collection of them. So this page skips the ‘choose’ page when
the user doesn’t yet have any contact blocks.

tests/app/main/views/test_service_settings.py

@@ -2169,26 +2169,39 @@ def test_add_letter_contact_when_coming_from_template(
     fake_uuid,
     mock_get_service_letter_template,
 ):
-    data = {
-        'letter_contact_block': "1 Example Street"
-    }
-
-    page = client_request.post(
+    page = client_request.get(
         'main.service_add_letter_contact',
         service_id=SERVICE_ONE_ID,
-        _data=data,
         from_template=fake_uuid,
-        _follow_redirects=True
+    )
+
+    assert page.select_one('.govuk-back-link')['href'] == url_for(
+        'main.view_template',
+        service_id=SERVICE_ONE_ID,
+        template_id=fake_uuid,
+    )
+
+    client_request.post(
+        'main.service_add_letter_contact',
+        service_id=SERVICE_ONE_ID,
+        _data={
+            'letter_contact_block': '1 Example Street',
+        },
+        from_template=fake_uuid,
+        _expected_redirect=url_for(
+            'main.view_template',
+            service_id=SERVICE_ONE_ID,
+            template_id=fake_uuid,
+            _external=True,
+        ),
     )
 
     mock_add_letter_contact.assert_called_once_with(
         SERVICE_ONE_ID,
         contact_block="1 Example Street",
-        is_default=True
+        is_default=True,
     )
 
-    assert page.find('h1').text == 'Set letter contact block'
-
 
 @pytest.mark.parametrize('fixture, data, api_default_args', [
     (no_sms_senders, {}, True),