Add GOV.UK Design System style back links

The Design System has standardised on back links being at the top of the
page, decorated with a small text-coloured arrow.

I think this makes more sense than having them at the bottom, because it
suggests, in some way, being able to go back before commiting to any of
the forms on the page. Whereas the things at the bottom of the page
should be performing actions on what’s in the page.

The reason for making this change now is that it de-clutters the area
around the green buttons. This was presenting a design challenge where
multiple levels of interaction were happening in the same form. Moving
these back links to the top of the page should mean that, in these
complicated forms, there’s one fewer thing to compete for the user’s
attention.

I’ve componentised this into a `page_header` macro so that the change is
easier to roll out and maintain.

tests/app/main/views/test_service_settings.py

@@ -2127,8 +2127,8 @@ def test_edit_reply_to_email_address(
     ),
     (
         get_default_reply_to_email_address,
-        'Back',
-        partial(url_for, '.service_email_reply_to'),
+        None,
+        None,
     ),
 ])
 def test_shows_delete_link_for_email_reply_to_address(
@@ -2148,10 +2148,18 @@ def test_shows_delete_link_for_email_reply_to_address(
         reply_to_email_id=sample_uuid(),
     )
 
-    last_link = page.select('.page-footer a')[-1]
+    assert page.select_one('.govuk-back-link').text.strip() == 'Back'
+    assert page.select_one('.govuk-back-link')['href'] == url_for(
+        '.service_email_reply_to',
+        service_id=SERVICE_ONE_ID,
+    )
 
-    assert normalize_spaces(last_link.text) == expected_link_text
-    assert last_link['href'] == partial_href(service_id=SERVICE_ONE_ID)
+    if expected_link_text:
+        link = page.select_one('.page-footer a')
+        assert normalize_spaces(link.text) == expected_link_text
+        assert link['href'] == partial_href(service_id=SERVICE_ONE_ID)
+    else:
+        assert not page.select('.page-footer a')
 
 
 def test_confirm_delete_reply_to_email_address(
@@ -2340,8 +2348,8 @@ def test_default_box_shows_on_non_default_sender_details_while_editing(
     ),
     (
         get_default_sms_sender,
-        'Back',
-        partial(url_for, '.service_sms_senders'),
+        None,
+        None,
     ),
 ])
 def test_shows_delete_link_for_sms_sender(
@@ -2361,10 +2369,20 @@ def test_shows_delete_link_for_sms_sender(
         sms_sender_id=sample_uuid(),
     )
 
-    last_link = page.select('.page-footer a')[-1]
+    link = page.select_one('.page-footer a')
+    back_link = page.select_one('.govuk-back-link')
 
-    assert normalize_spaces(last_link.text) == expected_link_text
-    assert last_link['href'] == partial_href(service_id=SERVICE_ONE_ID)
+    assert back_link.text.strip() == 'Back'
+    assert back_link['href'] == url_for(
+        '.service_sms_senders',
+        service_id=SERVICE_ONE_ID,
+    )
+
+    if expected_link_text:
+        assert normalize_spaces(link.text) == expected_link_text
+        assert link['href'] == partial_href(service_id=SERVICE_ONE_ID)
+    else:
+        assert not link
 
 
 def test_confirm_delete_sms_sender(
@@ -2389,8 +2407,8 @@ def test_confirm_delete_sms_sender(
 
 
 @pytest.mark.parametrize('fixture, expected_link_text', [
-    (get_inbound_number_sms_sender, 'Back'),
-    (get_default_sms_sender, 'Back'),
+    (get_inbound_number_sms_sender, None),
+    (get_default_sms_sender, None),
     (get_non_default_sms_sender, 'Delete'),
 ])
 def test_inbound_sms_sender_is_not_deleteable(
@@ -2409,8 +2427,14 @@ def test_inbound_sms_sender_is_not_deleteable(
         sms_sender_id='1234',
     )
 
-    last_link = page.select('.page-footer a')[-1]
-    assert normalize_spaces(last_link.text) == expected_link_text
+    back_link = page.select_one('.govuk-back-link')
+    footer_link = page.select_one('.page-footer a')
+    assert normalize_spaces(back_link.text) == 'Back'
+
+    if expected_link_text:
+        assert normalize_spaces(footer_link.text) == expected_link_text
+    else:
+        assert not footer_link
 
 
 def test_delete_sms_sender(
@@ -2607,12 +2631,17 @@ def test_request_letter_branding(
         **extra_args
     )
     assert request_page.select_one('main p').text.strip() == 'Your letters do not have a logo.'
-    link_href = request_page.select_one('main a')['href']
+    back_link_href = request_page.select('main a')[0]['href']
+    link_href = request_page.select('main a')[1]['href']
+    assert link_href == url_for(
+        'main.feedback',
+        ticket_type='ask-question-give-feedback',
+        body='letter-branding',
+    )
     feedback_page = client_request.get_url(link_href)
     assert feedback_page.select_one('textarea').text.strip() == (
         'I would like my own logo on my letter templates.'
     )
-    back_link_href = request_page.select('main a')[1]['href']
     assert back_link_href == expected_partial_url(service_id=SERVICE_ONE_ID)
 
 
@@ -3837,12 +3866,12 @@ def test_set_inbound_sms_when_inbound_number_is_not_set(
 @pytest.mark.parametrize('user, expected_paragraphs', [
     (active_user_with_permissions, [
         'Your service can receive text messages sent to 07700900123.',
-        'If you want to turn this feature off, get in touch with the GOV.UK Notify team.',
+        'If you want to switch this feature off, get in touch with the GOV.UK Notify team.',
         'You can set up callbacks for received text messages on the API integration page.',
     ]),
     (active_user_no_api_key_permission, [
         'Your service can receive text messages sent to 07700900123.',
-        'If you want to turn this feature off, get in touch with the GOV.UK Notify team.',
+        'If you want to switch this feature off, get in touch with the GOV.UK Notify team.',
     ]),
 ])
 def test_set_inbound_sms_when_inbound_number_is_set(