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.

app/jobs/avatar/avatar_from_gravatar_job.rb

@@ -0,0 +1,13 @@
+class Avatar::AvatarFromGravatarJob < ApplicationJob
+  queue_as :low
+
+  def perform(avatarable, email)
+    return if GlobalConfigService.load('DISABLE_GRAVATAR', '').present?
+    return if email.blank?
+    return if avatarable.avatar_url.present?
+
+    hash = Digest::MD5.hexdigest(email)
+    gravatar_url = "https://www.gravatar.com/avatar/#{hash}?d=404"
+    Avatar::AvatarFromUrlJob.perform_later(avatarable, gravatar_url)
+  end
+end