chore: Ability to Disable Gravatars (#5027)

fixes: #3853

- Introduced DISABLE_GRAVATAR Global Config, which will stop chatwoot from making API requests to gravatar
- Cleaned up avatar-related logic and centralized it into the avatarable concern
- Added specs for the missing cases
- Added migration for existing installations to move the avatar to attachment, rather than making the API that results in 404.

app/controllers/api/v1/accounts/callbacks_controller.rb

@@ -90,9 +90,7 @@ class Api::V1::Accounts::CallbacksController < Api::V1::Accounts::BaseController
   end
 
   def set_avatar(facebook_inbox, page_id)
-    avatar_file = Down.download(
-      "http://graph.facebook.com/#{page_id}/picture?type=large"
-    )
-    facebook_inbox.avatar.attach(io: avatar_file, filename: avatar_file.original_filename, content_type: avatar_file.content_type)
+    avatar_url = "https://graph.facebook.com/#{page_id}/picture?type=large"
+    Avatar::AvatarFromUrlJob.perform_later(facebook_inbox, avatar_url)
   end
 end