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.

spec/factories/inbox_members.rb

@@ -2,7 +2,7 @@
 
 FactoryBot.define do
   factory :inbox_member do
-    user
+    user { create(:user, :with_avatar) }
     inbox
   end
 end