Warn users a template change will break things

When a user adds or removes placeholders in their template we should consider
this a ‘breaking change’ and warn them accordingly.

Implementing this mostly relies on using
https://github.com/alphagov/notifications-utils/pull/37

Temporarily storing the new template until the user confirms that they want to
make the changes in done using hidden fields. This is a bit hacky, but the
complexity of making sessions interact with WTForms was just too much to handle.

This commit also changes the example spreadsheet that we show on this page to
look more like a spreadsheet.

tests/app/main/views/test_templates.py

@@ -1,5 +1,6 @@
 import json
 import uuid
+from bs4 import BeautifulSoup
 from tests import validate_route_permission
 
 from flask import url_for
@@ -67,6 +68,47 @@ def test_should_redirect_when_saving_a_template(app_,
                 template_id, name, 'sms', content, service_id, None)
 
 
+def test_should_show_interstitial_when_making_breaking_change(
+    app_,
+    api_user_active,
+    mock_login,
+    mock_get_service_template,
+    mock_update_service_template,
+    mock_get_user,
+    mock_get_service,
+    mock_get_user_by_email,
+    mock_has_permissions,
+    fake_uuid
+):
+    with app_.test_request_context():
+        with app_.test_client() as client:
+            client.login(api_user_active)
+            service_id = fake_uuid
+            template_id = fake_uuid
+            response = client.post(
+                url_for('.edit_service_template', service_id=service_id, template_id=template_id),
+                data={
+                    'id': template_id,
+                    'name': "new name",
+                    'template_content': "hello ((name))",
+                    'template_type': 'sms',
+                    'service': service_id
+                }
+            )
+
+            assert response.status_code == 200
+            page = BeautifulSoup(response.data.decode('utf-8'), 'html.parser')
+            assert page.h1.string.strip() == "Confirm changes"
+
+            for key, value in {
+                'name': 'new name',
+                'subject': '',
+                'template_content': 'hello ((name))',
+                'confirm': 'true'
+            }.items():
+                assert page.find('input', {'name': key})['value'] == value
+
+
 def test_should_not_create_too_big_template(app_,
                                             api_user_active,
                                             mock_login,