Add an endpoint to download a contact list

In case you need to download it, modify it, and re-upload it when you
don’t have access to the existing list.

app/main/views/uploads.py

@@ -3,7 +3,7 @@ import itertools
 import json
 import urllib
 import uuid
-from io import BytesIO
+from io import BytesIO, StringIO
 from zipfile import BadZipFile
 
 from flask import (
@@ -13,6 +13,7 @@ from flask import (
     redirect,
     render_template,
     request,
+    send_file,
     url_for,
 )
 from notifications_utils.columns import Columns
@@ -437,3 +438,14 @@ def contact_list(service_id, contact_list_id):
         'views/uploads/contact-list/contact-list.html',
         contact_list=ContactList.from_id(contact_list_id, service_id=service_id),
     )
+
+
+@main.route("/services/<uuid:service_id>/contact-list/<uuid:contact_list_id>.csv", methods=['GET'])
+@user_has_permissions('send_messages')
+def download_contact_list(service_id, contact_list_id):
+    contact_list = ContactList.from_id(contact_list_id, service_id=service_id)
+    return send_file(
+        filename_or_fp=StringIO(contact_list.contents),
+        attachment_filename=contact_list.saved_file_name,
+        as_attachment=True,
+    )