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/contact_spec.rb

@@ -2,6 +2,8 @@
 
 require 'rails_helper'
 
+require Rails.root.join 'spec/models/concerns/avatarable_shared.rb'
+
 RSpec.describe Contact do
   context 'validations' do
     it { is_expected.to validate_presence_of(:account_id) }
@@ -12,6 +14,10 @@ RSpec.describe Contact do
     it { is_expected.to have_many(:conversations).dependent(:destroy_async) }
   end
 
+  describe 'concerns' do
+    it_behaves_like 'avatarable'
+  end
+
   context 'prepare contact attributes before validation' do
     it 'sets email to lowercase' do
       contact = create(:contact, email: 'Test@test.com')