Add page explaining how to send template using API

Would like to test something like this and see how well it works.

Intention of having this page is so:
- template IDs are discoverable (https://www.pivotaltracker.com/story/show/115404593)
- it’s obvious there’s an ‘automated’ way to send messages, as well as the CSV
  way (we’ve seen people oblivious to this in research)

tests/app/main/views/test_send.py

@@ -90,6 +90,27 @@ def test_send_test_message_to_self(
         mock_s3_upload.assert_called_with(ANY, '12345', expected_data, 'eu-west-1')
 
 
+def test_send_test_message_from_api_page(
+    app_,
+    mocker,
+    api_user_active,
+    mock_login,
+    mock_get_service,
+    mock_get_service_email_template,
+    mock_s3_upload,
+    mock_has_permissions
+):
+    with app_.test_request_context():
+        with app_.test_client() as client:
+            client.login(api_user_active)
+            response = client.get(
+                url_for('main.send_from_api', service_id=12345, template_id=54321),
+                follow_redirects=True
+            )
+        assert response.status_code == 200
+        assert 'API integration' in response.get_data(as_text=True)
+
+
 def test_download_example_csv(
     app_,
     mocker,