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/agent_bot_spec.rb

@@ -1,5 +1,6 @@
 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 AgentBot, type: :model do
   describe 'associations' do
@@ -9,5 +10,6 @@ RSpec.describe AgentBot, type: :model do
 
   describe 'concerns' do
     it_behaves_like 'access_tokenable'
+    it_behaves_like 'avatarable'
   end
 end