Add a preview of basic view

One of the big things we found in user research was that people were
uncertain what the effect of giving someone basic view was.

So in the spirit of ‘show don’t tell’, this commit adds a way for users
to preview basic view. They can go into the preview and click around as
much as they like, just as if they really had the basic view assigned to
them.

Once they have seen enough they can return to the settings page where
they can decide whether or not to switch basic view on for real.

tests/app/main/views/test_service_settings.py

@@ -2406,6 +2406,10 @@ def test_see_basic_view_page(
         service_id=SERVICE_ONE_ID
     )
     assert page.h1.text.strip() == 'Basic view'
+    assert page.select_one('main a')['href'] == url_for(
+        'main.preview_basic_view',
+        service_id=SERVICE_ONE_ID,
+    )
     assert page.select_one('input[checked]')['value'] == expected_selected
 
 
@@ -2442,6 +2446,77 @@ def test_update_basic_view(
     ) == expected_updated_permissions
 
 
+def test_preview_basic_view(
+    client_request,
+    mock_get_service_templates,
+):
+    page = client_request.get(
+        "main.preview_basic_view",
+        service_id=SERVICE_ONE_ID,
+        _follow_redirects=True,
+    )
+
+    with client_request.session_transaction() as session:
+        assert session['basic'] is True
+
+    assert page.h1.text.strip() == 'Choose a template'
+    page.select('.navigation-service-basic-view-preview')
+    assert normalize_spaces(page.select_one('.navigation-service').text) == (
+        'service one '
+        'Preview of basic view '
+        'Back to settings '
+        'Switch service'
+    )
+    page.select_one('a.navigation-service-basic-view-back-link')['href'] == url_for(
+        'main.service_set_basic_view',
+        service_id=SERVICE_ONE_ID,
+    )
+
+    client_request.get(
+        "main.service_set_basic_view",
+        service_id=SERVICE_ONE_ID,
+    )
+
+    with client_request.session_transaction() as session:
+        assert 'basic' not in session
+
+
+def test_cant_preview_basic_view_for_another_service(
+    client_request,
+    mock_get_service_templates,
+    fake_uuid,
+):
+    client_request.get(
+        "main.preview_basic_view",
+        service_id=SERVICE_ONE_ID,
+        _follow_redirects=True,
+        _expected_status=200,
+    )
+    client_request.get(
+        "main.preview_basic_view",
+        service_id=fake_uuid,
+        _expected_status=403,
+    )
+
+
+def test_actual_basic_view_cant_escape_basic_view(
+    client_request,
+    active_caseworking_user,
+    mock_get_service_templates,
+):
+    client_request.login(active_caseworking_user)
+    client_request.get(
+        "main.preview_basic_view",
+        service_id=SERVICE_ONE_ID,
+        _expected_status=403,
+    )
+    client_request.get(
+        "main.service_set_basic_view",
+        service_id=SERVICE_ONE_ID,
+        _expected_status=403,
+    )
+
+
 def test_service_settings_when_inbound_number_is_not_set(
     logged_in_client,
     service_one,