Redo send_messages as create_and_send_messages

Reverts changes in the following commits:
- 1e8ba88b4d
- 133a62022c
- b76066dd76

These changes `main.create_and_send_messages` to
`main.send_messages` but this name was already
taken.

Further details on Github:

https://github.com/alphagov/notifications-admin/pull/3213#pullrequestreview-348548476

app/main/views/sub_navigation_dictionaries.py

@@ -66,7 +66,7 @@ def using_notify_nav():
             "sub_navigation_items": [
                 {
                     "name": "Send messages",
-                    "link": "main.send_messages",
+                    "link": "main.create_and_send_messages",
                 },
                 {
                     "name": "Formatting",