Add a ‘private’ preview email template endpoint

This will make working on the email template easier.

Includes examples of features like lists, headings, blockquotes, etc.

tests/app/main/views/test_email_preview.py

@@ -0,0 +1,23 @@
+import pytest
+from flask import url_for
+
+
+@pytest.mark.parametrize(
+    "query_args, params", [
+        ({}, {'govuk_banner': True}),
+        ({'govuk_banner': 'false'}, {'govuk_banner': False})
+    ]
+)
+def test_renders(app_, mocker, query_args, params):
+    with app_.test_request_context(), app_.test_client() as client:
+
+        mock_html_email = mocker.patch(
+            'app.main.views.index.HTMLEmail',
+            return_value=lambda x: 'rendered'
+        )
+
+        response = client.get(url_for('main.email_template', **query_args))
+
+        assert response.status_code == 200
+        assert response.get_data(as_text=True) == 'rendered'
+        mock_html_email.assert_called_once_with(**params)