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/inbox_spec.rb

@@ -2,6 +2,7 @@
 
 require 'rails_helper'
 require Rails.root.join 'spec/models/concerns/out_of_offisable_shared.rb'
+require Rails.root.join 'spec/models/concerns/avatarable_shared.rb'
 
 RSpec.describe Inbox do
   describe 'validations' do
@@ -37,6 +38,7 @@ RSpec.describe Inbox do
 
   describe 'concerns' do
     it_behaves_like 'out_of_offisable'
+    it_behaves_like 'avatarable'
   end
 
   describe '#add_member' do