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.

db/migrate/20220712145440_sync_gravatar_for_existing_avatarables.rb

@@ -0,0 +1,26 @@
+class SyncGravatarForExistingAvatarables < ActiveRecord::Migration[6.1]
+  def change
+    return if GlobalConfigService.load('DISABLE_GRAVATAR', '').present?
+
+    sync_user_avatars
+    sync_contact_avatars
+  end
+
+  private
+
+  def sync_user_avatars
+    ::User.find_in_batches do |users_batch|
+      users_batch.each do |user|
+        Avatar::AvatarFromGravatarJob.perform_later(user, user.email)
+      end
+    end
+  end
+
+  def sync_contact_avatars
+    ::Contact.where.not(email: nil).find_in_batches do |contacts_batch|
+      contacts_batch.each do |contact|
+        Avatar::AvatarFromGravatarJob.perform_later(contact, contact.email)
+      end
+    end
+  end
+end