Fix tests broken by GOV.UK Frontend template

Fix test broken by removal of GOV.UK logo image
- tests/app/main/views/test_notifications.py

Fix tests that ref GOV.UK template CSS files
- tests/app/main/views/test_index.py

Fix test broken by govuk-visually-hidden
- tests/app/main/views/test_send.py

Fix tests broken by header menu link -> button
- tests/app/main/views/test_service_settings.py
- tests/app/main/views/test_template_folders.py

Fix tests broken by removing div#content
- tests/app/main/views/accounts/test_choose_accounts.py
- tests/app/test_navigation.py

Remove references to GOV.UK CSS
- tests/app/main/views/test_index.py

Fix tests querying the header navigation
- tests/app/test_navigation.py

tests/app/main/views/test_send.py

@@ -94,7 +94,7 @@ def test_show_correct_title_and_description_for_sender_type(
 
     assert page.select_one('h1').text == expected_title
 
-    for element in ('legend', 'legend .visually-hidden'):
+    for element in ('legend', 'legend .govuk-visually-hidden'):
         assert normalize_spaces(page.select_one(element).text) == expected_description