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/builders/contact_builder.rb

@@ -23,7 +23,7 @@ class ContactBuilder
   end
 
   def update_contact_avatar(contact)
-    ::ContactAvatarJob.perform_later(contact, contact_attributes[:avatar_url]) if contact_attributes[:avatar_url]
+    ::Avatar::AvatarFromUrlJob.perform_later(contact, contact_attributes[:avatar_url]) if contact_attributes[:avatar_url]
   end
 
   def create_contact