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/models/user_spec.rb

@@ -2,6 +2,7 @@
 
 require 'rails_helper'
 require Rails.root.join 'spec/models/concerns/access_tokenable_shared.rb'
+require Rails.root.join 'spec/models/concerns/avatarable_shared.rb'
 
 RSpec.describe User do
   let!(:user) { create(:user) }
@@ -25,6 +26,7 @@ RSpec.describe User do
 
   describe 'concerns' do
     it_behaves_like 'access_tokenable'
+    it_behaves_like 'avatarable'
   end
 
   describe 'pubsub_token' do