chore: Move agent availability to Account level (#3074)

- Move agent availability to the account level

db/migrate/20210923190418_add_online_status_to_account_users.rb

@@ -0,0 +1,21 @@
+class AddOnlineStatusToAccountUsers < ActiveRecord::Migration[6.1]
+  def change
+    change_table :account_users, bulk: true do |t|
+      t.integer :availability, default: 0, null: false
+      t.boolean :auto_offline, default: true, null: false
+    end
+
+    update_existing_user_availability
+  end
+
+  private
+
+  def update_existing_user_availability
+    User.find_in_batches do |user_batch|
+      user_batch.each do |user|
+        availability = user.availability
+        user.account_users.update(availability: availability)
+      end
+    end
+  end
+end