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_template_folders.py

@@ -835,7 +835,7 @@ def test_delete_template_folder_should_request_confirmation(
     assert page.select_one('input[name=name]')['value'] == 'sacrifice'
 
     assert len(page.select('form')) == 2
-    assert len(page.select('button')) == 2
+    assert len(page.select('button')) == 3
 
     assert 'action' not in page.select('form')[0]
     assert page.select('form button')[0].text == 'Yes, delete'