input letter address data in a single block

rather than in multiple placeholders - this is the first step towards
making postcodes non-required, which is the first step towards
international letters.

they still populate address_line_# and postcode fields under the hood -
to keep validation working the same, the last line always goes into
`postcode`.

the form normalises whitespace, removes extra new lines, and enforces
that you have between three and seven lines.

if the letter repeats address placeholders further down (eg "Dear
((address_line_1))"), then it'll fill those in as well. It'll still
prompt you to fill them in, but they'll be pre-filled.

app/main/views/send.py

@@ -39,6 +39,7 @@ from app.main import main, no_cookie
 from app.main.forms import (
     ChooseTimeForm,
     CsvUploadForm,
+    LetterAddressForm,
     SetSenderForm,
     get_placeholder_form_instance,
 )
@@ -307,6 +308,11 @@ def send_test(service_id, template_id):
             return_to='view_template',
             template_id=template_id))
 
+    if db_template['template_type'] == 'letter':
+        return redirect(
+            url_for('.send_one_off_letter_address', service_id=service_id, template_id=template_id)
+        )
+
     return redirect(url_for(
         {
             'main.send_test': '.send_test_step',
@@ -329,6 +335,72 @@ def get_notification_check_endpoint(service_id, template):
     ))
 
 
+@main.route(
+    "/services/<uuid:service_id>/send/<uuid:template_id>/one-off/address",
+    methods=['GET', 'POST']
+)
+@user_has_permissions('send_messages', restrict_admin_usage=True)
+def send_one_off_letter_address(service_id, template_id):
+    if {'recipient', 'placeholders'} - set(session.keys()):
+        # if someone has come here via a bookmark or back button they might have some stuff still in their session
+        return redirect(url_for('.send_one_off', service_id=service_id, template_id=template_id))
+
+    db_template = current_service.get_template_with_user_permission_or_403(template_id, current_user)
+
+    session['send_test_letter_page_count'] = get_page_count_for_letter(db_template)
+
+    template = get_template(
+        db_template,
+        current_service,
+        show_recipient=True,
+        letter_preview_url=url_for(
+            'no_cookie.send_test_preview',
+            service_id=service_id,
+            template_id=template_id,
+            filetype='png',
+        ),
+        page_count=session['send_test_letter_page_count'],
+        email_reply_to=None,
+        sms_sender=None
+    )
+
+    form = LetterAddressForm()
+
+    if form.validate_on_submit():
+        session['placeholders'].update(form.as_address_lines_1_to_7_with_postcode)
+
+        placeholders = fields_to_fill_in(
+            template,
+            prefill_current_user=(request.endpoint == 'main.send_test_step'),
+        )
+        if all_placeholders_in_session(placeholders):
+            return get_notification_check_endpoint(service_id, template)
+
+        first_non_address_placeholder_index = len(first_column_headings['letter'])
+        return redirect(url_for(
+            'main.send_one_off_step',
+            service_id=service_id,
+            template_id=template_id,
+            step_index=first_non_address_placeholder_index,
+        ))
+
+    return render_template(
+        'views/send-one-off-letter-address.html',
+        page_title=get_send_test_page_title(
+            template_type='letter',
+            help_argument=None,
+            entering_recipient=True,
+            name=template.name,
+        ),
+        template=template,
+        form=form,
+        optional_placeholder=False,
+        back_link=get_back_link(service_id, template, 0),
+        help=False,
+        link_to_upload=True,
+    )
+
+
 @main.route(
     "/services/<uuid:service_id>/send/<uuid:template_id>/test/step-<int:step_index>",
     methods=['GET', 'POST'],