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_notifications.py

@@ -227,10 +227,9 @@ def test_notification_page_shows_page_for_letter_notification(
     letter_images = page.select('main img')
 
     assert len(letter_images) == count_of_pages
-
-    for index in range(1, count_of_pages + 1):
+    for index in range(count_of_pages):
         assert page.select('img')[index]['src'].endswith(
-            '.png?page={}'.format(index)
+            '.png?page={}'.format(index + 1)
         )
 
     assert len(mock_page_count.call_args_list) == 1