Add a setting to disable django messages

[KalobTaulien]

Currently when you save a snippet or Wagtail Page a Django Message will appear (either green for success or red for failure).

Models will still save on your site even if Airtable isnt updated.

In some cases you may not want to show two message bars to your editors:

A good first issue and enhancement would be to allow these to be turned off globally so only the regular Wagtail notification messages appear, and the Airtable notification messages are silenced.