Auto populate the phone number when replying

Currently the only way of ‘replying’ to an inbound message is by
copy/pasting the phone number from the inbound page into the send one
off page. Rather than making people copy/paste, this commit puts the
phone number in the session, if the user is going through the new reply
flow.

tests/app/main/views/test_conversation.py

@@ -257,8 +257,32 @@ def test_conversation_reply_shows_templates(
         assert normalize_spaces(link.text) == expected
         assert link.select_one('a')['href'].startswith(
             url_for(
-                'main.view_template',
+                'main.conversation_reply_with_template',
                 service_id=SERVICE_ONE_ID,
+                notification_id=fake_uuid,
                 template_id='',
             )
         )
+
+
+def test_conversation_reply_redirects_with_phone_number_from_notification(
+    client_request,
+    fake_uuid,
+    mock_get_notification,
+    mock_get_service_template,
+):
+
+    page = client_request.get(
+        'main.conversation_reply_with_template',
+        service_id=SERVICE_ONE_ID,
+        notification_id=fake_uuid,
+        template_id=fake_uuid,
+        _follow_redirects=True,
+    )
+
+    for element, expected_text in [
+        ('h1', 'Preview of Two week reminder'),
+        ('.sms-message-recipient', 'To: 07123 456789'),
+        ('.sms-message-wrapper', 'service one: Template <em>content</em> with & entity'),
+    ]:
+        assert normalize_spaces(page.select_one(element).text) == expected_text