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_service_settings.py

@@ -1187,8 +1187,8 @@ def test_non_gov_user_is_told_they_cant_go_live(
     assert normalize_spaces(page.select_one('main p').text) == (
         'Only team members with a government email address can request to go live.'
     )
-    assert page.select('form') == []
-    assert page.select('button') == []
+    assert len(page.select('form')) == 0
+    assert len(page.select('button')) == 1
 
 
 @pytest.mark.parametrize('consent_to_research, displayed_consent', (