fix: Index custom attribute uniqueness on account (#3516)

- Move custom attribute uniqueness validation to account

db/migrate/20211207113102_remove_attribute_key_index_on_custom_attribute_definition.rb

@@ -0,0 +1,7 @@
+class RemoveAttributeKeyIndexOnCustomAttributeDefinition < ActiveRecord::Migration[6.1]
+  def change
+    remove_index :custom_attribute_definitions, [:attribute_key, :account_id], name: 'attribute_key_model_index',
+                                                                               if_exists: true
+    add_index :custom_attribute_definitions, [:attribute_key, :account_id], unique: true, name: 'attribute_key_index', if_not_exists: true
+  end
+end