redirect from address_line_n to address textarea page

if someone starts a new one-off flow they'll get taken to the address
page. However, if someone hits the back button, they'll cycle backwards
through placeholders and will end up on the individual line pages. Lets
redirect them to the correct place.

We'll additionally need to reconstruct the address block from the
various session variables that may or may not be populated

tests/app/main/views/test_send.py

@@ -2015,66 +2015,34 @@ def test_send_test_caches_page_count(
         assert session['send_test_letter_page_count'] == 9
 
 
-def test_send_test_indicates_optional_address_columns(
+def test_send_one_off_back_link_populates_address_textarea(
     client_request,
     mocker,
     mock_get_service_letter_template,
+    mock_template_preview,
     fake_uuid,
 ):
-
-    mocker.patch('app.main.views.send.get_page_count_for_letter', return_value=1)
-
     with client_request.session_transaction() as session:
         session['recipient'] = None
-        session['placeholders'] = {}
+        session['placeholders'] = {'address line 1': 'foo', 'address line 2': 'bar', 'address line 3': ''}
 
+    # imagine someone hit the back button to go from line 3 page to line 2 page
     page = client_request.get(
         'main.send_test_step',
         service_id=SERVICE_ONE_ID,
         template_id=fake_uuid,
-        step_index=3,
-    )
-
-    assert normalize_spaces(page.select('label')[0].text) == (
-        'address line 4 '
-        'Optional'
-    )
-    assert page.select('.govuk-back-link')[0]['href'] == url_for(
-        'main.send_one_off_step',
-        service_id=SERVICE_ONE_ID,
-        template_id=fake_uuid,
         step_index=2,
+        _follow_redirects=True
     )
 
+    assert page.select_one('h1').text.strip() == 'Send ‘Two week reminder’'
 
-def test_send_test_allows_empty_optional_address_columns(
-    client_request,
-    mocker,
-    mock_get_service_letter_template,
-    fake_uuid,
-):
+    form = page.select_one('form')
+    assert form.select_one('label').text.strip() == 'Address'
 
-    mocker.patch('app.main.views.send.get_page_count_for_letter', return_value=1)
-
-    with client_request.session_transaction() as session:
-        session['recipient'] = None
-        session['placeholders'] = {}
-
-    client_request.post(
-        'main.send_test_step',
-        service_id=SERVICE_ONE_ID,
-        template_id=fake_uuid,
-        step_index=3,
-        # no data here
-        _expected_status=302,
-        _expected_redirect=url_for(
-            'main.send_test_step',
-            service_id=SERVICE_ONE_ID,
-            template_id=fake_uuid,
-            step_index=4,
-            _external=True,
-        ),
-    )
+    textarea = form.select_one('textarea')
+    assert textarea.attrs['name'] == 'address'
+    assert textarea.text == 'foo\nbar'
 
 
 def test_send_test_sms_message_puts_submitted_data_in_session(