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/index.py

@@ -357,9 +357,9 @@ def branding_and_customisation():
 
 
 @main.route('/using-notify/guidance/create-and-send-messages')
-def send_messages():
+def create_and_send_messages():
     return render_template(
-        'views/guidance/send-messages.html',
+        'views/guidance/create-and-send-messages.html',
         navigation_links=using_notify_nav(),
     )
 

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",